Make Branch-Filter Collapsable

[forgottosave]

Description

Make the branch filter collapsable. Branches starting with the same substring should be grouped (which could even be stacked over multiple layers) to reduce the branch-filter from a huge list to a good overview.

Example

If, e.g., multiple branches start with prod/ and others with dev/, they should be grouped. This would reduce the size of the view. One could also check/uncheck whole "folders" at once.
If a "folder" only has one child, there should not be a collapsable view for that.

☐ prod
☐ dev/one
☐ dev/two
☐ dev/two/test
☐ dev/three

would result in something like this

  ☐ prod
► ☐ dev/...

or expanded:

  ☐ prod
▼ ☐ dev/...
    ☐ two
    ☐ two/test
    ☐ three