Fixes for combo boxes - Githubissues

Summary:

CSoundPluginProcessor:
- "filetype" was used some places where "fileType" should have been. Fixed to use CabbageIdentifierIds::filetype.
- Made updates to CSound combobox channels use paths relative to the csd file
- Fixed an off-by-one error in which selection is pushed
- Removed the file extension for consistency with the rest of the codebase
- Fixed a bug where CSD_PATH was set incorrectly on Windows
CabbageComboBox:
- Made paths relative to the csd file
- Uncommented lines which send info to csound channels

rorywalsh / cabbage