A lot improvement in nexus.window and nexus.window.command. Also add 5 scenes corresponding to title menu.

 [NEW] Add nexus.scene.newgame.
 [NEW] Add nexus.scene.continue.
 [NEW] Add nexus.scene.option.
 [NEW] Add nexus.scene.extra.
 [NEW] Add nexus.scene.exit.
 [MOD] Now nexus.window.render will take visible attribute in account.
 [MOD] Add option and extra entries to nexus.scene.title.
 [MOD] Modify the internal index of nexus.window.command from string to
number to get correct order.
 [ENH] Improve nexus.window.update function.
 [ENH] Imrpove nexus.window.render function.
 [ENH] Improve nexus.scene.title update loop.
 [ENH] Now nexus.scene.title uses correct scene transfer.
 [DEL] Remove incomplete console statements in nexus.scene.stage.