WIP
Change the recent changes menu and file item dialog behavior

Recent Changes:
- "No items synced recently"
- Add separator to the details section
- Make items clickable (will open file location in file manager)
- If a file was deleted, disable it

File Item Dialog:
- Make items clickable (will open file location in file manager)

This is implemented via a new method Utility::showInFileManager(QString),
which goes through great lengths to use use the right file manager on all
OSes/WMs/DEs, and make sure it selects the file in question if possible.
This needs good testing, though.

Fixes #1044