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
`get_selected_item_text()` should always returns a string

```
Radus-MacBook-Pro:gtk3 radu$ pwd
/Users/radu/Kano/kano-toolset/kano/gtk3

Radus-MacBook-Pro:gtk3 radu$ ack selected_item_text
kano_combobox.py
82:        self.selected_item_text = ""
171:        self.selected_item_text = item.get_label()
183:    def get_selected_item_text(self):
185:        return self.selected_item_text
191:        self.selected_item_text = self.items[index]
222:        self.selected_item_text = ""
```