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
It may break however if `self.items[index]` is not actually a string... should I add a type check here [KanoComboBox L:202](https://github.com/KanoComputing/kano-toolset/blob/master/kano/gtk3/kano_combobox.py#L202) and  [KanoComboBox L:210](https://github.com/KanoComputing/kano-toolset/blob/master/kano/gtk3/kano_combobox.py#L210) ?