* moved the sprite renaming out of the file system entirely into a caller-provided callback.
* renamed several functions to closer match the terms of a file system.
* moved the VM interface out of the implementation.
This is now being managed by the main file system class. The single lumps should only concern themselves with the actual data they manage, not with how the file system presents them to the outside.
The IWAD detection code was also switched to use a file system wrapper instead of looking at the single files directly.
Precompilation of prefix header for GCC and Clang requires some efforts thanks to CMake which doesn't support this feature out of the box
Existing thirparty solutions must be tuned to our needs, and our configuration should be adjusted to a chosen module
If we ever want to refactor the global level data these must not reference the 'level' variable.
The main parts of the map loader cannot use this information, because it can only be created after running the node builder, so it got its own set of index functions instead.
