Implemented ability to open a directory as an archive (pretty basic for now).
Some things it doesn't do yet:
- Doesn't handle delete/rename of folders correctly
- Doesn't handle changes made to the directory outside of SLADE while it is still open
Merge branch 'master' of https://github.com/sirjuddington/SLADE
FluidSynth can now load several soundfonts

Also sound fonts and PNG tools now use their own dir_last.