Generic Wayland platform theme

Wrap generic platform theme around a Wayland implementation that
prevents platform themes for X11 desktop environments from being
loaded.

This won't affect desktops with Wayland support that correctly set
$XDG_CURRENT_DESKTOP or the now obsolete $DESKTOP_SESSION.

Change-Id: I0d683bfe491c9421acdb87a8e8f05d99de1ff21f