Add GLUON_DEFINE_SINGLETON macro for convenience.
Add GLUON_DEFINE_SINGLETON macro for convenience.

pranavrc/example-gluon,pranavrc/example-gluon,pranavrc/example-gluon,KDE/gluon,KDE/gluon,KDE/gluon,KDE/gluon,pranavrc/example-gluon