Jewels game

[cxong]

This branch adds the "jewels" game to the codebase.

It is activated by globally defining __JEWELS__, so without it the game is unchanged. The bare essentials of the game mode are implemented. Z key switch is replaced with WASD directional switching. This will need to be fully integrated later, perhaps as a different game mode accessed via the main menu.

[dorkster]

This looks cool, but it needs some work:

• Menu navigation is "broken", since the user should be able to press 'z' to confirm menu selections
• The control scheme in general is a bit weird. I don't like having two direction inputs for this style of game. The idea that I have is to split block switching into two stages. The first stage is triggered by pressing 'z'. This places another cursor on an adjacent block. The second stage allows the player to pick which adjacent block the cursor is on with the directional buttons. Finally, they can press 'z' to confirm the switch. Of course we can use the 'x' button to cancel out of the initial selection. It might be a bit slower, but it's more in line with the default game's control scheme.
• I too would like to make this a selectable game mode from the main menu. That means some preprocessor defines will have to be moved to actual variables.
• There should be a separate high scores list depending on the game mode.

I will work on tackling all of these issues.

[dorkster]

All right! I've merged your code and cleaned things up. You can now switch game type from the main menu. The control scheme is better than my original idea: press 'z' to enter switch mode, then a direction to switch in that direction. I also added some graphics to indicate which blocks can be swapped. Lastly, I made it so there are separate high scores files for each gametype.