GUI backbone tests

The tests in this subtree are a bit special: they cover the generic and
backbone internals of the Lumiera GTK GUI. They are linked against the
complete GUI-module (gui plugin), and thus may use all related ABIs.

Yet these tests are *deliberately* compiled without any GTK, GTKmm or SigC
includes. This effectively rules out the use, even indirectly, of any GTK
widgets and APIs -- forcing the covered GUI backbone entities to stay
clean and generic at API level.

This is a decision done on purpose. The concrete GUI framework technology
shall be treated as an implementation detail. There is no point in writing
tests which click buttons in the GUI -- better delegate any significant
logic or functionality to GUI agnostic components. GUI is meant to be
a presentation layer and must not develop intelligence on its own.