[Feature] Version management views should be split into two

[alessandrofelder]

Is your feature request related to a problem? Please describe.
There are many atlases and we plan to add more - but we expect users to typically only download and interact with a few of them. So it should be easier to distinguish the local and not-local atlases.

Describe the solution you'd like
The version management widget should contain two views, one for the locally available atlases (with a signal to show which ones can be updated) and one for the not-yet-downloaded atlases.

Describe alternatives you've considered
Could also colour-code rows, but this is both more difficult to implement and less clear to users, I think.

Additional context
May be worth refactoring the code to use proxy models instead of different views in this process?