GdkEvent has been a "I-can't-believe-this-is-not-OOP" type for ages,
using a union of sub-types. This has always been problematic when it
comes to implementing accessor functions: either you get generic API
that takes a GdkEvent and uses a massive switch() to determine which
event types have the data you're looking for; or you create namespaced
accessors, but break language bindings horribly, as boxed types cannot
have derived types.
The recent conversion of GskRenderNode (which had similar issues) to
GTypeInstance, and the fact that GdkEvent is now a completely opaque
type, provide us with the chance of moving GdkEvent to GTypeInstance,
and have sub-types for GdkEvent.
The change from boxed type to GTypeInstance is pretty small, all things
considered, but ends up cascading to a larger commit, as we still have
backends and code in GTK trying to access GdkEvent structures directly.
Additionally, the naming of the public getter functions requires
renaming all the data structures to conform to the namespace/type-name
pattern.
We want to make events readonly, so stop translating
their coordinates and instead pass the translated
coordinates separately, when propagating events.
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.
