Always have Since: annotations at the very bottom, use the correct
ClassName::signal-name/ClassName:property-name syntax, fix a few typos
in type names, wrong function names, non-existing type names, etc.
gtk_gesture_drag_get_start_point and gtk_gesture_drag_get_offset
have out args that need to be annotated.
This commit adds the (out) and (nullable) annotations as appropriate.
https://bugzilla.gnome.org/show_bug.cgi?id=734292
This prevents some of our generic object implementation tests
from working with gesture objects. Instead, add g_return_if_fail
checks in all the gesture constructors.
