Add user application data directory to JSON model paths

[usdivad]

This PR adds the user application data directory to the list of paths to search for when loading JSON models as dropdown choices, with the goal of addressing #3. Specifically, the plugin will search for <userAppDataDirectory>/GuitarML/SmartPedal, based on the plugin manufacturer and plugin name set in the project .jucer.

The plugin will automatically create (and delete) a temp file in the directory if it doesn't exist yet, so that users don't have to manually create the directory -- however, users will have to reinstantiate the plugin after adding model files to the directory for those models to show up in the dropdown.

SmartPedalAudioProcessorEditor::modelSelectChanged() has also been adjusted to use selectedItemIndex instead of selectedText, in order to work with paths that aren't in the current working directory.

Potential further TODOs:

• Add File::commonDocumentsDirectory as a path as well
• Add a "Re-scan models" button so users don't have to restart the plugin after copying a model over
• Create an installer and have that do the directory setup (instead of doing it in the plugin processor constructor)

[GuitarML]

Thanks for the PR! Seems to fix the issue and also a good idea to have a common area for models.