Use lowercase for tracks dir name.

This is for consistency with the other directory names.

This only causes compatibility issues for Linux (since Windows filenames
are case-insensitive), but since we never released a Linux version of
1.24, this will only affect developers.
Move bundled tracks to share dir.

Since we load a random track for the main menu, we now have the bundled
tracks already in the destination directory so the game will work nearly
out-of-the-box.

The source files for the tracks have been moved from res/tracks/mazes to
res/tracks to line up.
Enable all bundled tracks for the main menu.

Now that the bundled tracks are in the right place straight out of the
repo, we can choose from all four of them when picking a random track
for the main menu background.