header file changes to catch up with gtkmm-2.24.3

Some changes are needed to compile against a recent version of gtkmm in use by homebrew, see mxcl/homebrew#19416 and #7.  With these changes, I can compile YARP just fine using the current library versions in homebrew (also removing a dependency on glademm).

@drdanz, the change to gtkdatabox would more appropriately be made upstream, right?