smarter detection of when sprites can be moved

For the move commands, a pre-move delegate was added to the constructor
to raise NotifyPropertyChanging events for the VM's CanMoveUp and
CanMoveDown, and the post-move delegate was expanded to raise
NotifyPropertyChanged events.  The CanMoveUp and CanMoveDown properties
were changed to check the index of the current sprite in the collection.
adding icons to UI elements
consolidating more styles for current sprite UI
consolidating styles for sheet fields/sprite list
swapping label/image placement in sprite buttons