make library and plugin dependencies declarative

instead of directly including the respective pri files in
*_dependencies.pri, set variables and let qtcreator.pri resolve
them to includes.
this will allow us to re-use the dependency info elsewhere.

Change-Id: Iaa33924e428ac0409660f42df2f98a7978452d3e
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>