[UX] Allow switching the menu being used for a menu block.

Say for whatever reason, a site builder needs to replace a menu with another via the admin UI, while at the same time retaining the configuration (menu style and depth, style settings, visibility conditions):

1. Add a new menu block in the same region.
2. [add as many steps/clicks here as required to manually copy the configuration of the old menu block...]

...

n-2. ... n-1. Remove the old menu block n. Save the layout

...so basically the more complicated configuration the old menu block has, the more clicking a site builder needs to do in order to replicate the block configuration to the new menu block.

Compare the above to this:

1. Edit existing menu block.
2. Select another menu from a drop-down menu that lists all available menus.
3. Save the menu block
4. Save the layout