gtkdnd: Don't bind/unbind keycodes that couldn't be determined

gtkdnd: Don't bind/unbind keycodes that couldn't be determined

At http://dev.laptop.org/ticket/10643 we are seeing that drag-and-drop
within the Sugar shell causes all of Sugar's custom keybindings to be
removed.

This is because gtkdnd tries to unbind XK_KP_Space, which (on my systems)
is resolved to NoSymbol by XKeycodeToKeysym(). NoSymbol has value 0,
the same as AnyKey, and XUngrabKey(AnyKey) is equivalent to unbinding
all possible keycodes.

Fix this by catching NoSymbol before binding/unbinding.

https://bugzilla.gnome.org/show_bug.cgi?id=652402