Added ViewportGadget raster<->world space conversion methods.
Simplified GafferUI::View bindings using IECorePython::CastToIntrusivePtr.
Added Handle::dragOffset() method.

This can be called from a dragMove() slot to obtain the current offset in the direction of the handle.
Merge pull request #950 from johnhaddon/handleDragOffset

Bits and bobs of progress towards manipulators