break devil's circle:

the MonoDevelop/Xamarin GUI designer tool "Stetic" needs existing (!) binary compilations of our custom widgets when it runs.

however, these only exist when our build is completed.

to break this devil's circle, create trivial dummy implementations of them which can be added to Stetic's toolbar
so that its designer and build process can happily doing their work.
in the real, final build, these classes will not be used from the dummy library "DummyWidgetPlaceholdersForStetic", but instead their real implementations.

Change-Id: I5b0dfca3136f6fb4d376674a7f1674a774c0435c