Validate slots on startup

[NotCoffee418]

Ensure previously configured slots are still valid on startup.
If not, request that user runs the config again & cancel startup.

This can occur if user changed a screen's position or removed a screen.
Slot.IsNewLocationValid() and/or TryInitialize needs to be modified

[NotCoffee418]

Use WpfScreen to validate slots.
Also use it in SlotConfigWindows instead of whatever i have now.