feature-request(settings): make setting for default sf2 path

[gaabora]

Hello. Thank you for such an awesome plugin!
Would be nice to have option for default sf2 files path, so when you select a file it's path got saved related to that directory (if it's inside of that directory)
The purpose for the setting is to have cross-platform compatibility for the plugin's settings,
for example: I use the same Reaper project in windows and macos, and have to manually change the path of sf2 file each time I switch between systems.

Thank you!