Removed Windows-specific code.

Replaced Windows Properties with simple accessors.
platform-specific UI calls still need to be inserted in their own
partials.
Factorized some platform-independent initialization.
Added constructors that assign the added and removed items lists.

Otherwise the class would be useless. InvokeEventHandler is an override
of its parent, btw.
ToString must return string, not be void.
Added default constructor and empty implementation of OnSetSource.
Removed more Windows-specific code and uncommented code that is now compatible.

By implementing a longListSelector, now a bunch of code that was
commented because of incompatibility is now usable.
Removed code that had to do whit XAML templates and the designer.
Init renamed to InitPicker because it conflicted with NSObject's.
A proposal for the generic picker (abstract class) for MonoTouch.

It compiles, at least. Won't be tested until FriendPicker is ready for
testing.