right now we have to defeat an unfortunate static assertion in
the standard library, which is expected to go away in the future.
We use a hack to hijack the problematic definition with the preprocessor,
which requires our header to be first.
- split off the tricky basic GTKmm includes
- reduce the clutter in the gtk-lumiera.header
- make GtkLumiera a singleton, placed into static memory
- remove all static functions from WindowManager
