frontend: allow sharing collapsible containers between Views

[ES-Alexander]

Current behaviour

#1689 adds cloning behaviour to Collapsible Container widgets, but it's somewhat unintuitive to use, requires going to a different View in order to add an element to the View you're actually trying to modify, and results in independent clones that need to be synchronised manually whenever one of them has its contents changed.

Expected or desired behaviour

Per this comment:

Could we have them linked by their name (per #1527), instead of cloning by default?

I think that would generally be more intuitive than needing to use a special menu on a widget, in a View you're not trying to change, to add something to a different View. Having an intuitive linking key would also allow keeping changes between them in sync automatically (which is likely usually desirable).

The interface for that could be similar to GitHub's branch filtering/naming dropdown, where an empty, unnamed [Collapsible Container] would have a search bar for the name, with a label like "Find or create a Collapsible Container...", then when you type a name it filters by existing ones, and if nothing matches exactly then there's a button to "Create a new Collapsible Container called XX".

If we want to also support [unlinked] cloning then that could instead be performed when you rename one that already has mini-widgets in it.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.