Allow adding additional plugin directories per format

[dromer]

Currently there is only a single directory configured for each plugin format, however it is very common to have several locations.

On Linux and VST2 there are for instance:

• /usr/lib/vst - for plugins installed via package manager
• /usr/local/lib/vst - for third party packages installed manually
• ~/.vst - for manually installed plugins inside the users home directory

[dromer]

Ok my bad, I didn't realize you can easily add additional directories :)