gtktextdisplay: Optimize GtkStyleContext usage
gtkstylecontext: Optimize GtkStyleContext usage
gtklabel: Optimize GtkStyleContext usage
gtkentry: Optimize GtkStyleContext usage
x11: Handle obsolete selection requests without crashing

The ICCCM says:

If the specified property is None , the requestor is an obsolete client.
Owners are encouraged to support these clients by using the specified
target atom as the property name to be used for the reply.

Lets do that, instead of crashing.

https://bugzilla.gnome.org/show_bug.cgi?id=740613