removes unused import

so scumbag created a commit just for changing a line.
jokes away, as usual, they recommend doing each thing in a
different commit, so that if things go wrong and you need to
revert changes, you don't revert unexpected files.
Bullet spawning code is refactored (not at all)

Bullet spawning code has moved from the listeners to the screen.
This is not a full refactor: a context stage (extend Stage class
and add all the stuff that the level requires) is planned,
this will cause bullet spawning code to be moved to the context
too.
Aliens and bullets nomore fly over UI

All UI elements are now part of a group. The group is added to
the stage. This way, there's an unique node in the stage direct
children, making easier to manage the UI elements.

Aliens and bullets are added before UI group on stage.