gtk: Run GtkWidgetClass event signals inside a GtkEventController

gtk: Run GtkWidgetClass event signals inside a GtkEventController

This will allow further cleanups and optimizations in capture/target/bubble
event delivery. For simplicity, ATM every widget will receive its own
GtkEventControllerLegacy, it could be desirable to add finer control over
this in the future, so widgets that fully use event controllers for input
management can do away without this legacy piece.