GTK: Use the proper GTK3 way to set widget window background
GTK: Use the proper GTK3 way to set widget window background

It also drops use of gtk_style_context_get_background_color() that has
been deprecated in 3.16.

This however has no functional changes.

R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone