Update Blank app MainPage

[dansiegel]

What would you like to be added:

For the blank preset we currently just provide a MainPage in the project root directory. This exposes a simple UI with a "Hello Uno Platform" TextBlock. This should be updated to use a basic counter API.

When creating a project using the blank preset, but enabling either MVVM or MVUX we should additionally provide a ViewModel/Model that supports binding to the Updated Counter UI.

NOTE When Maui Embedding is also enabled the Counter UI from Uno should be hidden as we already have a Counter UI in the EmbeddedControl.