[FIX] Enabling logic of ProjectOptionComposite was fragile

Instead of checking which button of the two radio buttons ("Create new"
or "Use existing") triggered an "updateEnablement", now the resulting
selection is checked.
Where this should not make a big difference for a human user, the
previous behavior broke a couple of STF test cases.

Change-Id: I4dffffde11dbf9bbe05e3b168b2d88c934c21c0a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1520
Tested-by: Jenkins CI
Reviewed-by: Maria Formisano <mformisano87@gmail.com>
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>