Add option for applauncher to respond to the super key

[fossfreedom]

Should be pretty straightforward. If you look at the budgie menu applet in the budgie desktop git repo you see that it responds to an action event that is called when the super key is pressed.

We should consider adding that as an option so users can dispense entirely with the budgie menu applet but still have the same response to the super key to open

[serdarsen]

I started to research about the super key.

[Jacob-Vlijm]

Do we see applauncher (still) as an appropriate candidate to fully replace the menu applet? In other words, is this still something we'd want?