Set preferred angle units for symbol styles

[eurojam]

Feature description.
Allow the user to set the preferred angle units to radians, gon (same as measure tool) for the rotation values in the layer styling widget. This can be important when using data defined rotation angle which comes not in degrees and will be used together with the rotate Symbol tool from the advanced digitizing toolbar. This will write wrong values into the data field in case the rotation values stored in the field are not in degrees.