Added Drag and drop support to move F# project items within the same project

Added Drag and drop support to move F# project items within the same project.  Dragging non project items and dragging out of project results in default base behaviour.  
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This should address #135 for the most part.  I don't know whether adding an Up/Down entry to the context menu would help make this more visible.  Im sure somebody could add this at a later date, dragging entry's seems a lot more intuitive.

Added more Linq2Xml helpers to TooltipHelpers to facilitate drag and drop, also refactored so they are in their own module.