[UX] Layouts: Allow menu "source" to be changed from within any menu block

Use case:

• You have configured a specific menu block the way you want it (menu style, depth, style/classes, visibility conditions etc.)
• You want to replace that menu with another, but retain the respective block settings.

In order to achieve this currently, you have to go to the layout(s) where this menu block is added, add a new menu block, and manually copy the settings to it from the existing block; then finally remove the old menu block. This is just too many clicks and really bad UX.

Proposal: Add a "Menu source" drop-down menu within each menu block, to allow changing the menu this block renders; keep all other menu settings.