Remove the src directory from search path.

 [DEL] Remove the src directory from search path
 The default vsync is enabled to limit FPS at 60.

 [MOD] The default vsync is enabled to limit FPS at 60.
 A new skeleton of screen.

[NEW] A new skeleton of screen.
[MOD] Modify nexus.screen.title to use new skeleton.
* Note: The draw call of screens will be removed and managed in
nexus.manager.screen.