glade: Fix macro renamed in GladeUI 3.37
Glade 3.37 renamed the macro GWA_GET_CLASS to GLADE_WIDGET_ADAPTOR_GET_ADAPTOR_CLASS 1. This commit switches to use the new macro name, and for older versions of Glade, defines the new name as the old one for compatibility.
Fixes #335 (closed) and obsoletes !607 (closed).