copitmizations: Fix mistake with pycairo header import

https://bugzilla.gnome.org/show_bug.cgi?id=715125
Allow disable help
Updated Czech translation
bin: Handle gi typelib path
[l10n]Updated Turkish translation
system: Use the system instance for displaying notifications
system: When the notification daemon is dead get over it
system: Fix debug statement
system: Remove System._listToString
system: A bit more Pythony
Get rid of assert False
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
Remove unused imports and fields
medialibrary: Reuse the code for setting the dragged paths
medialibrary: Fix supported file formats lists
medialibrary: Use a local variable instead of a field for the error dialog box
medialibrary: Pythonic check
medialibrary: Allow importing MTS files