Add a continuous-gesture-aware stack widget
We need to make applications using GtkStack able to have a continuous go back gesture like presented in the followin video.
As GTK+ 3 is now frozzen, we probably could implement it as a fork of GtkStack which enables some gestures and bind them to its transition animation. For simplicity of the API, the available gestures could depend on the set animation, which would make porting from GtkStack easy and merging that feature upstream in GTK+ 4 easy too.
When the gesture is finished, if more that half of the current child is visible we should trigger moving back to it completely, otherwise we should move to the other child.
We need a way to bind the progression of the gesture between multiple widgets, for example for a stack in the title bar to follow the movement of the stack in the window.