Rework the build system to better use parallel builds.

Instead of having one gigantic master .pro, split things out into multiple
SUBDIRS templates. This allows for slightly better grained build dependencies
without losing the sanity of a clearly understandable build system.

Change-Id: Ib09cd6170bcf8be55a1b351731f9a6451bb82d27
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>