Luca Bacci authored Aug 03, 2018 and Mike Fleetwood committed Feb 11, 2019

Now that we are compiling against Gtkmm3 there are missing declarations
of Gtkmm identifiers due to changes in Gtkmm internal header structure.

All we have to do is bring back the declarations by including the
appropriate headers where needed.

Add necessary Gtkmm header includes.

Closes #7 - Port to Gtk3