Various

[Jopyth]

This PR depends on cginternals/qmltoolbox#31.

• Introduces internal stages (stages existing only in the UI, not in the actual pipeline)
• The former preview item was converted to an internal PreviewStage
• The preview item currently can be connected to slots of type globjects::texture and gloperate::ColorRenderTarget with type UserDefinedFBOAttachment
• Added: information on required slots and feedback connections
• Fixed: changing the pipeline through the UI also updates the editor correctly

Bugged:

• Preview stages have wrong dimensions/data in the new "external" window mode, however this would often be used with a second display, and the preview stages might not be needed in that case anyway, therefore they are currently hidden in "external" mode

Missing:

• It would be nice to automatically connect the (first) PreviewStage to the same output which is connected to the ColorOut slot of the a pipeline.
• Support other RenderTargets and types of render targets