This also improves visuals in the Voxel and Dynamic Split Screen demos,
and fixes splitscreen mode in the Platformer 2D demo.
Most demos (other than physics tests) have also had their physics FPS
increased to 120. This is done consistency with 4.x, but also for the
benefits it still provides when interpolation are enabled
(lower input lag, more stable simulation).
Opened them, saved all scenes, ran the project. Did necessary fixes here and there,
fixed some warnings (but some remain in bigger demos).
Did not port JRPG demo yet (#306) nor the two platformer demos.
Now there's a base state_machine script, the StateMachine is separate from the physics body.
I reworked the code to follow the GDscript guidelines in the official manual.
