Favourite label presets

[mzur]

Users asked to be able to configure more favorite labels. It can work like this: There are up to 10 sets of favorite labels. By default, set 0 is active and new favorite labels are added to this set (current behavior). But now users can switch to a different set by pressing Ctrl+0-9. Switching to a different empty set effectively clears the favorite label tree and allows the user to add another 10 favorite labels.

In the UI this should be shown with a dropdown button next to the favorite label tree title. In the button, the current number of the selected label set is shown. Users can click this and get a dropdown to select another label set. Favorite label sets should be remembered in local storage similar to favorite labels now.

This was a user request.

I close the idea with virtual label trees in favor of this one.