meta-xfce: use bb.utils.contains() instead of base_contains()

base_contains() is a compatibility wrapper and may warn in the future, so
replace all instances with bb.utils.contains().

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>