Move non-BrainGlobe-specific Qt elements to a separate NIU repository

[niksirbi]

Is your feature request related to a problem? Please describe.
This issue is following up on a discussion we started on zulip.

The gist of the problem is that brainglobe-utils currently hosts some Qt elements, like CollapsibleWidgetContainer that are (or can be) useful for projects beyond BrainGlobe. The first example of that is the movement napari plugin, which depends on the aforementioned element. Depending on brainglobe-utils means also getting other stuff that movement and similar projects won't need - e.g. some BrainGlobe-specific dependencies, and the brainmapper widget.

While this is not critical yet, it may soon reach a breaking point, especially if more repos (NIU or otherwise) start re-using those Qt elements for their own sake.

Describe the solution you'd like

• Identify all non-brainglobe-specific Qt elements
• Move those to a new neuroinformatics-unit/qt-widgets repository (name subject-to-change)
• Make brainglobe-utils and movement depend on that new repository.

Describe alternatives you've considered
Leave things as they are, as nothing is broken, yet.

Additional context
This was recently re-brought to my attention thanks to @lochhh noticing that brainmapper appeared among available napari plugins when using movement.