xdg-shell: Reword the xdg_wm_base introduction

Don't refer to things as "traditional desktop" as it is not defined
nor clear what that refers to; instead reword things in a more explicit
way. A reason for this is that xdg-shell is not strictly meant only for
traditional window stacking based desktop environments, but should be
equally suitable for stacking, tiling and potentially other styles as
well.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-By: Mike Blumenkrantz <zmike@osg.samsung.com>