some hardcore refactoring of the settingsportal

sits on top of !171 (merged) so that probably should go in first...

this gives the settings portal a modular architecture so we can easily add more settings handlers in the future. most notably we'll need to add org.kde.TabletMode and org.kde.VirtualKeyboard to represent the relevant kwin settings/states. These are not backed by kdeglobals so it'd be silly to mix them in with the rest of the KDE handling. To that end I'd like them to be their own settingsmodule.

the end result is that the actual portal implementation only works with a vector of SettingsModule instances and delegates work as necessary