Started adding new tutorial world, world 0. First level complete.
Added new tutorial levels.
Adding dumb_stunner to index.html
Added difficulty switch button to world map state.

Will remove challenge mode button and replace with practice mode button in
future CL.
Finished adding practice button as a separate mode.

Also fixed background transitions from practice mode to level intro state
and game over state.

Hard mode now has its own special background when you toggle.
Changed quests to be called challenges.

Fixed bug where clicking on START GAME or PRACTICE before the buttons
faded in will still cause the bg to change even though the game state
doesn't change.
No longer zoom out when player dies.
Fixed some bugs relating to when player first starts playing.

Removed Practice button if it's player's first_time.

Level no longer spawns enemies if the player has died.
Added alternate title backgrounds. Subject to change.