Added utility functions for interacting with the OpenMW vfs

find_file locates the file that OpenMW would load (last matching file in the data load order).

ci_exists determines if a file exists in a case insensitive manner, returning the actual path of the file if it exists.

Note that I don't know how OpenMW handles ambiguous paths (e.g. when both A/b/C and a/B/c exist, or when A/b/C exists and a/B exists, but a/B/C does not). ci_exists may incorrectly return false if part of the path is ambiguous.

This will be necessary to automate tes3cmd clean, since we need the installed locations of the masters.