Secure plugin loading

This PR stops invalid QML files crashing MuseScore via Plugin Manager and prevents them being marked as loadable at startup.

Faulty plugin scripts are highlighted in red in plugin manager and the "load" checkbox cannot be marked as checked for these scripts.

I can't find any examples of tests for dialogs, modal or otherwise, so haven't been able to work out how to write any.
Do we really to store this list in preferences? Looks like we could just use the existing pluginList no?