Fixup the windowmanager extension code

ie. don't have it in its own dir

+ fixup some errors in the string matching from when extensions where
renamed from a wl_ prefix to a qt_ prefix

Change-Id: I5707c24fa727b9e49bd267eeb23fc8e43c40354f
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>