Make UI plugins in order to choose your own style

[coenm]

RepoM supports plugins to be enabled/disabled to choose your own functionality. Currently, this is only backend code but not UI related.

This issue is about supporting plugins with UI in RepoM.

[coenm]

@gabriel-vanca did a lot of great work in #184 and #162. This issue is about moving these nice ui options into plugins

[gabriel-vanca]

Not sure if plugins is the best option for this as it could be difficult to maintain different UIs and UI technologies. #184 enables support for custom themes and colour palettes. Should be fairly easy to implement and anyone could design their own theme by just editing the colours in the relevant XAML dictionary file. We could also enable support for custom fonts, font sizes, wrap/no wrap, element heights etc. Those could just be loaded from the main settings file.
Maybe a better suggestion would be to split the settings file into two: one solely for the UI, one for the app functionality.