We no longer set the widget on construction, but instead require an
explicit call to gtk_widget_add_controller().
This way, the reference handling becomes explicit and bindable.
Because gtk_widget_add_controller() is (transfer: full), we don't
even need to unref the controller after adding it.
And we don't need to keep track of it, because controllers get cleaned
up by GtkWidget.
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.
