Project64: Improve flexibility of advanced mode settings toggle

[cennis91]

Extends the flexibility of advanced mode by allowing CSettingsPage to define it's own visibility/accessibility. This removes explicit handling of CAdvancedOptionsPage and CDefaultsOptionsPage in CSettingConfig.

This also opens the door for other conditional logic in displaying setting pages, if desired.

Fixes #2369

Does this make breaking changes?

No, but COptionsDirectoriesPage was not implementing CSettingsPageImpl and I couldn't see a reason why not, so I also changed that.

Does this version of Project64 compile and run without issue?

Yes