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.
There are a couple of things about this.
Ffirstly I was trying to disturb the existing code as little as possible, particularly given the current release status.
Secondly, I think the list is kept because there is a difference between the list of available plugins and the list that is funnable from the plugin menu.