Add a continuous-gesture-aware revealer widget
We need to add a way to hide the DTMF tone pad of Calls with a nice continuous gesture and a widget enabling that would fit libhandy.
As GTK+ 3 is now frozzen, we probably could implement it as a fork of
GtkRevealer which enables some gestures and bind them to its revealing animation. For simplicity of the API, the available gestures could depend on the set animation, which would make porting from
GtkRevealer easy and merging that feature upstream in GTK+ 4 easy too.
When the gesture is finished, if more that half of the child is visible we should trigger revealing it completely, otherwise we should hide it completely.
We need a way to bind the progression of the gesture between multiple widgets.