player: Add micro-libraries with GTK/Qt/QML/Android/iOS/OSX/Windows integration

Submitted by Sebastian Dröge @slomo

Link to original bug (#765318)

Description

For QML there is code here: https://github.com/sdroege/gst-player/tree/master/qt

For GTK there is code here: https://github.com/sdroege/gst-player/tree/master/gtk

For Android there are JNI bindings here: https://github.com/sdroege/gst-player/tree/master/android but these would ideally be autogenerated (see https://github.com/sdroege/gst-player/issues/39 )

This is more of a tracker bug and finding a general plan for handling these, specific platforms should be handled in new bugs.