Modified Reticle System, Added bullet system

* Reticle automatically spawns when an enemy comes into range (same as
TargetButton)
* Reticle automatically focuses on next enemy when currently focused
enemy is destroyed
* Reticle can still be scrolled through when TargetButton is pressed
* (BUG) First click on TargetButton re-creates reticle on the same unit
before scrolling
* (To Be Implemented) Left-Click on enemies should refocus the reticle
to the clicked enemy
* (To Be Implemented) Button to SHOOT should appear when an enemy has a
reticle on them (and shoot the bullet when clicked)

* Bullet can be spawned to destroy enemies when <Enter> is pressed (will
be changed into button, see above)
* Bullet cannot be spawned when a reticle doesn't exist