VST state file search

[jpcima]

This adds a mechanism lo let VST find files on a different environment, after the session has been shared/moved/etc.
It will establish a list of directories where SFZ files are expected to be stored.

When a file is missing after state loading, the plugin is going to run a background scan, synchronously but shared between multiple instances.
This will collect the paths into a map [filename]->[list of paths] with lower-cased keys.

On scan completion, the file path is looked up into the index, yielding a list of candidates.
Then, we study the absolute path by comparing directory components in reverse order with the candidate, yielding a score according to the number of matched components.

This process applies to sfz files as well as tuning files.

[jpcima]

There is other potential for optimization from here.

• speed: from experience fts.h is much faster for recursive file scanning on non-Windows, but the comparison was to boost::filesystem, not ghc
• memory: use a hierarchical file-trie structure such that directory components of absolute paths are not duplicated when stored