Added convenience methods for the Level Handler.

A convenience method for initializing the Gun and Targets and all related
attributes have been added to avoid code duplication.

Placeholder methods for each level have also been added.
Added functionality for the Next button in the LevelFinishScreen

The Next button is used to conveniently proceed to the next level right
away without having to go back to the Level Select Screen, provided the
Player has hit at least 1 target.