Replaced Gtk ComboBoxes with custom one https://github.com/KanoComputing...

*Replacing Gtk ComboBox with KanoComboBox*

 - replaced in `set_keyboard`
 - replaced in `set_display`
 - added 3 images in `media` for widget
 - minor change in paths

**Requires appropriate styling**
https://github.com/KanoComputing/kano-settings/issues/39
are you sure `get_selected_item_text()` will always return a valid country?