window: Only check for the event widget if clicked on the "content" region

window: Only check for the event widget if clicked on the "content" region

Every button press/release event reaching the the multipress gesture in GtkWindow
and happening in the "title" region must be handled, regardless of the event widget.
Children there wanting the event(s) for themselves are (and were always) expected
to stop event propagation.

So the only place to check for the event widget's "window-dragging" style property
is the "content" region, which matches the pre-gestures behavior.

This fixes some issues with sequences being mistakenly claimed (and events not
propagated further) on situations it shouldn't.