[UX] Layout UI: Collapsible block groups.

I've been reading the various issues people have recently opened about problems they are facing while trying to work with translation of various things in order to build multilingual sites and I honestly feel their frustration. Things are already hard enough to achieve in D7 with the help of contrib modules like l18n and its ecosystem of modules, but it will be even harder and more frustrating in Backdrop since i18n has not been ported yet.

I'm not even sure if l18n is the right solution for us since we moved to config. You see, when i18n started off, it was designed with having in mind as granted whatever was planned for D6/D7 and that did not include CMI back then. Perhaps we'll head to a different direction in Backdrop and that might be totally irrelevant or incompatible with i18n. So taking into consideration the size and complexity of the i18n module, it might not even be worth porting it to Backdrop at all.

The workarounds that we have so far: - For site names and slogans for example, is to remove the default header block that comes out of the box with each layout and then create multiple custom blocks (one for each language) in its place making sure that each custom block has a properly configured visibility (language). - For handling multilingual menus, the best solution currently is also to create one menu per language and then having one block per language-specific menu with properly set visibility conditions (language).

In websites with 2 or 3 languages this might not be a big deal, but when reaching close to a dozen languages, that makes the Layout UI really cluttered and just horrible to work with. Just imagine all the scrolling required to reach the footer region with so many blocks placed in the header (times 2 because you have language-specific header blocks and also the same amount of menu blocks). And it gets even worse when someone needs multiple menus in their site (#1690).

Anyways, until Backdrop 2 starts being a reality (that's when we plan to extend the language support to menus and other things like the site language), we should at least make the life of multilingual site builders a tad easier. So, can we please have a way to group multiple blocks together and somehow collapse them so they don't get in the way all the time. That way one could create a "Main menus" group of blocks that would include all the language-specific menu blocks they have created and a "Header blocks" group for their language-specific custom header blocks.

I was thinking that these could perhaps work the way you can group icons on iPhones/iPads and on Android phones by dragging a block in another block instead of dragging it to another region. In that case, after releasing the block, a dialog would pop up asking for a block group name.

I realize that my proposal here is in order to solve a problem that will be moot once we have proper language support for menus and header etc., but that will take a long time while people using Backdrop need to create multi-language websites now. Besides, I'm hopping that this feature here will be useful in other use cases too.

Thoughts? (especially @quicksketch and @pablo-romero)