Allow simple arithmetic in visual designer inspector fields

The visual designer inspector should accept simple arithmetic in supported numeric property fields. For example, if a control's Left value is 80 and the user wants to move it 10 pixels, they could type 80 + 10 and press Return; the inspector would evaluate the expression and store 90.

This would make quick layout adjustments easier for properties where arithmetic is meaningful, such as position, size, spacing, and other numeric pixel values. Unsupported fields should continue to behave normally and should show clear validation if the expression cannot be evaluated.