button: don't be active when holding the mouse button outside the bounds

button: don't be active when holding the mouse button outside the bounds

GtkButton currently draws itself as active (pressed down) in case we're
pressing and holding the mouse pointer outside its bounds; this is
misleading though, since we won't activate the button unless the mouse
is released inside the button itself.
Fix this by only setting the ACTIVE state flag when the button is
actually pressed down.

https://bugzilla.gnome.org/show_bug.cgi?id=668141