[UX] Either get rid of MENU_SUGGESTED_ITEM or change the way it works in a way that fixes UX WTFs

I initially started typing this in a comment in #5674, but realized that I might be derailing the issue (which aims to fix some menu-related issues when upgrading from D7 - which makes that either a task of a bug), so I am starting this new issue here (which is more of a feature request).

Why was MENU_SUGGESTED_ITEM introduced?

So let me see if I've got this right... it seems to me that MENU_SUGGESTED_ITEM was invented in order to hold "parked" menu items. So a module can use it to "suggest" menu items, which will only be available when that module is enabled. However, the difference with other module-provided menu items is that: 1. These items are not for admin-related tasks, so they are not to be added in the admin bar 2. We needed a way to have such menu items in a disabled state initially, so that each site owner could decide: - whether the suggested menu parent is the right one, and if not, then move the menu item to another parent (which could be an entirely different menu than the suggested one). - whether they even need that menu item in the first place (and if not, then keep it disabled - there is no way to delete these items anyway).

Is that it? It sounds like some legacy thing, that was implemented that way at a time when no other way was available or thought of to do things in a better, more user-friendly manner.

FWIW, MENU_SUGGESTED_ITEM was removed in D8.1: https://www.drupal.org/project/drupal/issues/1013726 ...but then again, the old routing system was removed in D8 anyway. So this is most likely irrelevant to what we are trying to do here.

Current UX WTFs

It seems to me that we are currently making many assumptions, which may lead to WTFs depending on what the site builder/owner wants to actually achieve. Consider the following scenarios (there can be many more, I'm sure): - Oh, that module I just enabled created a menu item for me (a non-admin one that is - it is not in the admin bar). Cool that I don't have to figure out default paths etc. ...but: - I just randomly discovered it. How am I supposed to know that it even exists? - Why was it placed in the main navigation menu? I don't want it there - Why is it disabled?
- WTF?! The menu item I had in that menu just vanished ...did disabling that module the other day have something to do with this? (this is the current behaviour with the PR available for #5674) - OK, I re-enabled that module. ...nice! That menu item was restored in the menu I had it, enabled, and in the order I had it. Backdrop is smart! (again, this is what happens with the PR in #5674 - thank you @indigoxela ) - Decided that I don't need the functionality of the module. I disable the module that provided that menu item, and I then completely uninstall it. ...nice! the menu item it provided is now gone ...clear caches (all or just menu) ...some time goes by, people either forget about that module and the menu item it provided, in what parent menu it was moved etc. because a lot of time has passed since, or maybe this is now a different site admin/developer - not the same person that had originally enabled, configured, and then disabled and uninstalled the module ...I need that module again, so I re-enable the module -> look for the menu item in the main nav menu, like the documentation for that module suggests -> menu item is not there. WTF!

Proposed solution/PoC

All that sound like an interesting UX problem and I would like to work on and provide a PoC with a few suggestions/ideas, which may solve some of these WTFs in what I think would be a better way...

So, if we are adding a MENU_SUGGESTED_ITEM menu item when enabling a module, how about we: - Don't place it in any specific existing menu (empty parent - don't make assumptions). We instead add it in some sort of "preconfigured" or "suggested" menu items library. People then are given the option to either "+ Add custom menu item" (create new) or "+ Place existing menu item" (menu items provided by modules via hook_menu()) - This new "module-provided menu item library" (whatever we want to call it) that is either under Structure -> Menus, or under Reports -> Module-provided menu items, does the following: - Shows me in which menu each such MENU_SUGGESTED_ITEM item is currently placed, and also provides handy links to jump to that menu - If a MENU_SUGGESTED_ITEM menu item is unused (was never placed anywhere, so it still has an empty parent), the link says "place menu item". - Have hook_menu() throw a status message when the module it enabled for the first time, along the lines of "The xyz module added the following menu item(s) ...[list of menu items + links to edit the menus that hold them - or jump to the suggested menu items library]" - Similarly, when disabling a module that provides a menu item, we throw a message (via confirm_form() or otherwise), to let people know that disabling the module will also remove the menu item(s) provided by it (if they are placed in a menu, and if they are not disabled).