Beginnings of EventSystem, UI, and some key stuff

Implementing the beginnings of the event system. Sample event and how it
may operate is in TestAIController and will activate when the enemy is
killed. See commented section. The sample dialogue event can optionally
pass another vent to be triggered when the dialogue box is closed. This
allows for chaining another event after the dialogue box has finished
its text script.

Early implementation of DialogueBoxes. For now, dialogue boxes will not
freeze game state, will add once I flesh out the design for dialogue
boxes a little more.

Changed the rendering logic. Each rendering module now contains its own
sprite batch so that batch begin calls no longer interfere with one
another (reduces coupling and enables safe use of stage for UI and other
stuff).

Added test assets for UI