Intial support for package paths (as suggested by smls).

This commit is mainly intended to ease packaging for linux.
Unless environment variable AQUARIA_DATA_PATH is set, there are two
directories which are checked by the game:
If AQUARIA_DEFAULT_DATA_DIR is defined, it will chdir there for main
operation. If it's not defined, it chdirs into the directory where
the executable is located.
Then, if AQUARIA_EXTRA_DATA_DIR is defined, it will mount this directory
and all contents into the working path, so that the files present there
will override those from the working directory when accessed by the game.

Setting the environment variable AQUARIA_DATA_PATH will disable this
behavior altogether and use *only* AQUARIA_DATA_PATH as working dir.
Use more sane defaults for CMake
More win32 cmake fixes, should now hopefully compile out of the box
Add support for "override" data directory (as suggested by smls).

Now it loads in this order:

AQUARIA_DEFAULT_DATA_DIR
AQUARIA_DEFAULT_DATA_DIR/override
AQUARIA_EXTRA_DATA_DIR
Seems this line was lost...