Add “Show in Finder” context menu when no items are selected

When you control-click the empty part of the file browser and select “Show in Finder” a Finder window will open showing the currently selected folder.

You can go about this other ways, but I find that this feature complements the already existing "New Folder" menu item in the no-items-selected context menu of the file browser.

This patch is public domain.
Make OakPasteboardSelector accessible

Issues remaining:

* VoiceOver for some reason cannot track keyboard focus
  (i.e. moving down the list with just down arrow does not move
  VoiceOver cursor). User has to move the VO cursor instead using VO-down
  (and keyboard cursor follows in this case)
* the pasteboard window isn't focused automatically for VoiceOver, user
  has to switch manually to it using VO-F2-F2
Make OakTabBarView accessible
Optimize path::disambiguate

We now sort the reversed paths and do a linear sweep.
Don’t link to GitHub’s download section

This is an attempt of satisfying issue #677 — not entirely sure what was intended by opening that issue (given that nothing is presently broken and I surely will figure out another place to upload the builds once GitHub’s service is shut down completely).
Update dialog plugins

Fixes #584
Show in Finder works for multiple selected items
Add folder’s display name to menu items

This is only for the menu which shows when there is no selection.
Exclude gutter from accessibility

This was read as “empty scroll view” (using voice over), so better to skip it.