ColorSpace bindings for QML/Quick

Qt 5.14 added a QColorSpace and QColorTransform type, unfortunately they only have C++ api so for, and need bindings to access colorspace correcting in QML.

I imagine three changes:
Add color-space read to Image node
Add builtin color-space conversion to Image node (with a target color-space)
Add a generic color-space transform node with source and target color-space that can be inserted anywhere in the scene-graph.

The first one requires read-access to a QColorSpace wrapper. The second can be done with existing QColorTransform. But the last one requires generating shaders to the various backends and might be better to wait for Rhi for.