Extend gitignore

After just building OMW, I see a number of untracked files (enough to fill a terminal and make it hard to see my actual changes). The first commit removes everything that was added during the build. The second commit is less important, but it organizes `.gitignore` in hopes of making it a bit less of a dumping ground and a bit more readable.

If we're taking in the second commit, reading the diff sucks, so just see it at my revision: https://github.com/jeaye/openmw/blob/gitignore/.gitignore