Add missing newline to configure summary output
Remove trailing spaces
Add missing includes
Make mate_gdk_spawn_command_line_on_screen available on GTK2
Correctly support GDK2 and GDK3 without a deprecation warning
Improve comments
Remove unneeded includes
Use G_{BEGIN,END}_DECLS instead of #ifdef __cplusplus in header files and make sure to include glib.h for these macros