the context can now override a virtual method named `CreateHelpContent` which the setup wizard system will automatically try to use when you use the generic overload taking a generic context type. If the return is null, it skips setting entirely (the default impl is null)
additionally made the discord join link a button with code copied from the about window, and made it centered at the bottom.
i.e. if you had keys installed, previously it'd skip right to firmware.
additionally added more customization to the now instance-based NotificationHelper
- Remove polymorphic base, this only existed because TKMM has a desktop/switch setup prodecure difference and has 2 implementations of the setup wizard. We only need one.
- Remove Systems/UI file split, they're all in Ryujinx.Ava.UI now
- made NotificationHelper instance-based to allow you to encapsulate notifications to a window that magically disappear when the window is closed, instead of switching to showing on the main window.
renamed view models to contexts (like TKMM), however the contexts here are actually of a unique base type; containing aforementioned setup step logic. if the return value is of an error state result, it will prompt a retry of the page.
