Allow unlimited number of folders in Preferences->Folders->{Styles,Templates,Plugins}

In preferences currently, we can specify an indefinate number of folders for soundfonts, but we are only allowed to specify one folder for Styles, Templates, and Plugins. I can understand why Scores & Images should only be single folder, since mscore will write output there. But it is conceivable that user may want multiple folders for Styles, Templates, and Plugins, which are all places mscore primarily reads.

One use case is if user is on development version of mscore, but wants to include folder for templates from regular release, then it would be nice to specify both in path. Or maybe user downloads a collection of templates from the internet but wants to keep in a seperate folder from personal. Or maybe user a wants to include templates from user b's home directory.