gtk: Port to GtkApplication
gtk: Port to GtkApplication

And also clean up some other things.

https://github.com/sdroege/gst-player/issues/56

JimmyOhn/gst-player,guiseco/gst-player,justinjoy/gst-player,jabiers/gst-player,sdroege/gst-player,sdroege/gst-player,sdroege/gst-player,guiseco/gst-player,shakin/gst-player,justinjoy/gst-player,justinjoy/gst-player,amorenoc/gst-player,guiseco/gst-player,amorenoc/gst-player,codomania/gst-player,codomania/gst-player,amorenoc/gst-player,sdroege/gst-player,shakin/gst-player,sdroege/gst-player,JimmyOhn/gst-player,JimmyOhn/gst-player,amorenoc/gst-player,codomania/gst-player,jabiers/gst-player,JimmyOhn/gst-player,shakin/gst-player,amorenoc/gst-player,JimmyOhn/gst-player,codomania/gst-player,codomania/gst-player,jabiers/gst-player