Multiple esm esp

This is the current state of the branch for multiple plugin support, offering the following features:

- Load multiple master/plugin files, as defined in the config file/selected in the launcher. The code will now verify that all dependencies are fulfilled independent of the launcher.
- Add, overwrite and remove records by plugins.
- Add, modify and delete references that have been defined by parent plugins. This includes moving references across cell boundaries.
- Implement multiple terrain palettes, one palette per plugin.
- Some hotfixes for more advanced data merging problems that will be addressed later.

Since I won't be able to work on it for the forseeable future due to Real Life complications (as already told on the forum), it should enter the main branch now; the code seems to be sufficiently stable in doing the above points. (I should still be able to do minor requests, it's just new stuff that I don't have the energy to do right now.)