Add boolean properties, is-pointer-focus and
contains-pointer-focus, that track whether the pointer
is in the widget itself or in one of its descendants.
The ::enter and ::leave signals get emitted up and down the
connecting path between the old an the new pointer location.
The signals are less useful if you can't find out where along
the path you are. That is what crossing mode and detail are
about, so add those to the signals.
The idea is that GTK+ 4 will be an epoch, API-wise.
Everything that was around for 4.0 has been there
since the beginning of the epoch and doesn't need
markers.
Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.
