dogtail tests: Use the filechooser's path togglebutton instead of the "/" key

This removes a trick where we simply typed the "/" character to force open
the path entry in GTK filechooser. We now check the path togglebutton's state
and click it if needed. This approach is potentially safer.

While theoretically slower, the actual performance impact is mostly negligible:
for example, the medialibrary test takes 20.8 seconds instead of 20.7.
dogtail tests: Add some sleeps to prevent false alarms on some tests
Update the app icon

- part of the gnome goal:
  https://wiki.gnome.org/GnomeGoals/HighResolutionAppIcons

https://bugzilla.gnome.org/show_bug.cgi?id=711382