TouchPanelCapabilities (IsConnected, MaximumTouchCount) for all platforms

I've implemented figuring out the TouchPanelCapabilities for each platform.
At the moment they are all #if'd in to the file, happy to tidy this up if you can suggest how :)

TouchCollection.IsConnected is also correctly implemented now, it is set based on the capabilities, rather than always saying true.

Fixes #2531
Fixes #2532