Add preliminary saving/loading of scene/sources

This saves scenes/sources from json on exit, and properly loads it back
up when starting up the program again, as well as the currently active
scene.

I had to add a 'load' and 'save' callback to the source interface
structure because I realizes that certain sources (such as scenes)
operate different with their saved data; scenes for example would have
to keep track of their settings information constantly, and that was
somewhat unacceptable to make it functional.

The optional 'load' callback will be called only after having loaded
setttings specifically from file/imported data, and the 'save' function
will be called only specifically when data actually needs to be saved.

I also had to adjust the obs_scene code so that it's a regular input
source type now, and I also modified it so that it doesn't have some
strange custom creation code anymore.  The obs_scene_create function is
now simply just a wrapper for obs_source_create.  You could even create
a scene with obs_source_create manually as well.