Added a CoordinateSystem node to GafferScene.
Implemented drawing of coordinate systems in the viewport.

Unfortunately this continues the abuse of the SceneProcedural, but that is inevitable until we have a more sophisticated viewer.
Added coordinate system output functions to RendererAlgo.h.
Added coordinate system support to ExecutableRender.
Added coordinate system support to InteractiveRender.

This is just preliminary support and doesn't have unit tests yet. Currently it only works if the Renderer::coordinateSystem() call in IECoreRI::Renderer is modified to use RiCoordinateSystem() instead of RiScopedCoordinateSystem().
Added unit tests for moving coordinate systems during IPR.

This requires 3delight 11.0.96 to pass successfully.
Added history tracebacks to the SceneInspector.

These are available from the right click context menu for any value displayed in the the SceneInspector.

Fixes #834.
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 #946 from johnhaddon/coordinateSystems

Coordinate systems
Merge pull request #950 from johnhaddon/handleDragOffset

Bits and bobs of progress towards manipulators
Changed shutdown warnings to debug messages.
Merge pull request #949 from johnhaddon/sceneInspectorHistory

Added history tracebacks to the SceneInspector.