Introduce priorities for build configuration factories. This way
plugins can register specialized build configuration factories, that
e.g. can provide additional build steps.

A negative priority signifies that a factory is not prepared to
handle a request, the default build configuration factory shipped by
the build system plugin will report a priority of 0. Add 100 to that
for each specialization you add (e.g. a remote linux buildconfiguration
factory would report 100, a specialization of that for mer will
should report 200, etc.).

Change-Id: I141a7a5a79166afdb7657d46eb7e86bd18d3abf6
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Michal Klocek <michal.klocek@digia.com>