Version 0.6. Background

- Added background to the Game Scene.
- To differentiate between backgrounds in possible scenes in the game,
they have a name and id system.
- Added feature to the mouse: clicking will spawn an enemy at the
mouse's position.

KNOWN BUGS:
- Sometimes, one enemy becomes inaccessible.
- 2nd background image goes fast than the 1st. In the first parallax the
1st image spawns correctly, but there is a gap because the 2nd going
faster.