notebook: return TRUE for drag-motion event when over tabs
notebook: return TRUE for drag-motion event when over tabs

The GtkNotebook drag-motion event handler may install a timeout when
hovering over a tab, in order to switch to it.
On the other hand it's desirable for applications to use the empty tab
area as a drop target, so the drag-motion handler returns FALSE
(also in case it installs the switch tab timeout), as explained in
https://bugzilla.gnome.org/show_bug.cgi?id=350665.

Unfortunately, applications can use the tab label widget (or a child
of it) as a different drop target area, and install their own
drag-motion handler there.
In this scenario, the timeout will still be installed by GtkNotebook's
handler, but since it returns FALSE, it will never get the matching
drag-leave event, causing it to trigger also when the mouse pointer
moved elsewhere before it expired.

Fix this by returning TRUE from drag-motion when the event is over a
tab. Note that this makes automatic tab switching not work anymore when
drag and drop is handled in the tab label widget; applications are
expected to also handle tab switching if desired in such a case.

https://bugzilla.gnome.org/show_bug.cgi?id=684415

jadahl/gtk,msteinert/gtk,ebassi/gtk,ebassi/gtk,jessevdk/gtk,jessevdk/gtk,ebassi/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,Distrotech/gtk2,davidt/gtk,jigpu/gtk,ebassi/gtk,grubersjoe/adwaita,grubersjoe/adwaita,Distrotech/gtk2,grubersjoe/adwaita,Distrotech/gtk2,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,davidt/gtk,jigpu/gtk,Adamovskiy/gtk,ahodesuka/gtk,grubersjoe/adwaita,ahodesuka/gtk,msteinert/gtk,chergert/gtk,msteinert/gtk,Lyude/gtk-,jessevdk/gtk,Adamovskiy/gtk,davidt/gtk,Lyude/gtk-,msteinert/gtk,ahodesuka/gtk,alexlarsson/gtk,Distrotech/gtk2,jessevdk/gtk,jadahl/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,bratsche/gtk-,davidt/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,Adamovskiy/gtk,Distrotech/gtk2,alexlarsson/gtk,ahodesuka/gtk,Lyude/gtk-,msteinert/gtk,Adamovskiy/gtk,davidgumberg/gtk,davidgumberg/gtk,bratsche/gtk-,chergert/gtk,chergert/gtk,Lyude/gtk-,jadahl/gtk,bratsche/gtk-,grubersjoe/adwaita,Adamovskiy/gtk,alexlarsson/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,davidt/gtk,jadahl/gtk,bratsche/gtk-,Lyude/gtk-,ahodesuka/gtk,bratsche/gtk-,chergert/gtk,jadahl/gtk,Adamovskiy/gtk,davidt/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,jadahl/gtk,jadahl/gtk,ahodesuka/gtk,chergert/gtk,chergert/gtk,ebassi/gtk,chergert/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,chergert/gtk,ebassi/gtk,davidgumberg/gtk,Adamovskiy/gtk,ahodesuka/gtk,jadahl/gtk,davidgumberg/gtk,davidgumberg/gtk,Lyude/gtk-,bratsche/gtk-,jessevdk/gtk,Distrotech/gtk2,jessevdk/gtk,jessevdk/gtk,grubersjoe/adwaita,msteinert/gtk,alexlarsson/gtk,davidgumberg/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Lyude/gtk-,jigpu/gtk