Reworked LevelManager, now supporting LoadLevel request for network
games.

Added GameManager prefab containing a LevelManager component.

Added CinematicIntro behaviour, which is simply a slideshow of textures
to be displayed before loading the next level.

Added dummy texture for intro slide show.
Updated intro scene to use new CinematicIntro component.

And now the GameManager will be instantiated dynamically at run time if
needed.

No need for timed trigger to start the next level, CinematicIntro will
trigger it at the end of the slide show.
Updated GameManager prefab.
Added simple MainMenu component with a panel of buttons, which either
trigger a level load or quit the game.

'main_menu' scene now uses this new component.
Added MainMenuEditor which provides custom inspector view for MainMenu component.
It provides the list of levels available in the current build settings to choose from.
Small changes in MainMenu.
TODO: need to change menu creation to be datadriven, editable from editor and possibly in game (only in a "special dev mode").
Added GameCamera abstract class, and first implementation of a third person camera.
Added base for GamePlayerController.
Added bases for GameTemplate and GameEntitySpawner.
Added base for player manager and networking.