Implemented tile-based walking

Tile-based collisions, talking, etc. implemented. A basic depth
controller (sets all objects' depth to -y) implemented.

Known Issues:
When starting a textbox, the character is "paused." However, while in
this paused state, Game Maker stores a single button press and executes
it immediately after unpause. So if you hit "right" during the dialogue,
your character will move right afterward.