Fixes for LongListMultiSelector.

Including:

LongListMultiSelector Adding to "SelectedItems" adds 2 entries
http://phone.codeplex.com/workitem/10822

No ScrollTo method on LongListMultiSelector
http://phone.codeplex.com/workitem/10782

Fixes to make LongListMultiSelector more closely match the built-in
experience.

And other minor fixes.