Enhance SLD Editor

[AlexanderSchober]

The SLD editor could benefit from more overlay information on hover on different objects. This would go hand in hand with refactoring the LayerComponents.

Other possible improvements:

• Make the color of SLD elements semi-transparent to reveal the SLD curve beneath.
• Make roughness handles visible when sigma=0.0
• Cleanup LayerElementItem from P_SIDE_THICKNESS and similar. They can be simple member variables.
• Consider accumulating constants appearing in expression P_TOP_PEN_WIDTH=5 in single place and make them dependent on WidthOfLetterM method for better High DPI experience.
• While reploting, make min/max of SLD editor +/- 10%. Introduce SLDCanvasItem solely for that purpose. See CanvasItem as an example.