Add STK-Mallets support for OSX

[tresf]

MacPorts does not seems to have a libstk-dev0 package so this would require downloading, compiling, copying headers and linking libstk from here:
https://ccrma.stanford.edu/software/stk/download.html

Further investigation shows libstk also depends on a "rawwaves" directory which must be accessible via LMMS as well, so this will need to be manually bundled into the Apple package and then changed within the configuration dialog to represent the correct relative path (configuration dialog shown)

[tresf]

Closed per #638