Ability to start overlay from the Configuration application

[hardworkingbox]

Would be a nice QoL change, bit weird that we can close the overlay but not start it.

[trigg]

Solid point, worth looking into

[trigg]

For now I've removed the button from Overview as it was added as a necessity when working with early SteamOS, where you could not close out non-steam games from the gamescope UI.

This is not really the case now.

Not closing this as it might still be worthwhile having a start option on the context menu