Moves Transitions package to engine-core. Slightly improves some transitions.
Adds Slice and None transitions.
Changes TransitionManager so it can be used within the engine.
Changes Moakp to use the new TransitionManager. Fixes other small bugs.
Adds transition enum to goscene.json.
Changes GoSceneExecutor to use the new TransitionManager.
Adds TransitionsDemo.
Merge pull request #857 from RotaruDan/engineTransitions

Engine transitions