Restructure the getters for event fields to
be more targeted at particular event types.

Update all callers, and replace all direct
event struct access with getters.

As a side-effect, this drops some unused getters.

Events are refcounted structs, and we generally don't
pass these as const.

GTK is no longer relying on this.

We can now get the target widget from the gesture
that we are using to find the event in the first
place.

We already store the events; keep the target widget
in addition. This is a step towards getting rid of
gdk_event_get_target.

Make it possible for event controllers to obtain
the target widget during handle_event.

Pass the event propagation target explicitly down to
the event controllers. This is a step towards getting
rid of gdk_event_set_target.

We are now sending crossing events (which are the only ones
where a related target makes sense) via handle_crossing.

The event propagation limit should apply to crossing events
as well.

Instead of relying on gdk's antiquated crossing events,
create a new GtkCrossingData struct that contains the
actual widgets, and a new event controller vfunc that
expects this struct. This also saves us from making sense
of X's crossing modes and details, and makes for a
generally simpler api.

The ::focus-in and ::focus-out signals of GtkEventControllerKey
have been replaced by a single ::focus-change signal that
takes GtkCrossingData as an argument. All callers have
been updated.

This is no longer used.

This is just a thin wrapper around gdk_event_get_target,
so use that directly.

We want to make events readonly, so stop translating
their coordinates and instead pass the translated
coordinates separately, when propagating events.

Events of type GDK_NOTHING are good for nothing.

Use gdk_event_ref/unref instead of g_object_ref/unref.
Events will stop being object soon.

No backend is emitting GDK_DESTROY events anymore, so no
need to carry this around.

We don't have child windows anymore, so there is no difference.

Use the new event constructors when generating events
for emulating presses.

At the same time, stop setting child_surface in
crossing events. Nothing in GTK looks at it.

Closes #1013

Instead of a fake one where we add .menu to a normal popover. This makes
the menu look like other context menus.

Otherwise it doesn't know what to do with its child widget.

This is triggered by typing / or hitting Ctrl+L. Since we don't have a
visual indicator for this mode right now anyway, the reason for not
allowing it in recent mode cited in the comment just above the
early-exit is irrelevant.

Closes #2178

Unused.

Everything is automatically redrawn when changing the value, etc.

We add the .menu css class to the popover main node, not its contents
node.

This brings us closer to the old GtkMenuItem behavior and also makes
more sense.