The actor allocation will be invalid on CLUTTER_TOUCH_BEGIN, because
it comes together with a CLUTTER_ENTER event that will recalculate
styles, and queue a relayout in result.
The net result is that on CLUTTER_TOUCH_BEGIN, the relayout has been
already queued, so the slider width comes up as 0, and the value ends
up as 1. Later touch events already happen on a validated actor, so
it is corrected. Still, not fun when modifying the volume slider on a
touchscreen.
We are already emitting a 'drag-end' signal when no more dragging
is happening, so it makes sense to emit a 'drag-begin' too when
starting, so that apps interested in implementing different logic
between those two events can easily do it without needing to deal
with the underlying 'button-press-event' signal for the actor.
https://bugzilla.gnome.org/show_bug.cgi?id=761208
This is a regression from splitting the slider out that never got fixed.
Restore the previous (useful) behavior by adding a public API to the
slider that lets us pass an event through.
https://bugzilla.gnome.org/show_bug.cgi?id=704368
