Fix line properties in property dialog (issue #488)
To fix issue #488,
- a clip rectangle is set on the line style and line arrow arrow property widget area
- the property widget image is rendered with the target window background
- and the rendered property widget image is paint in the window with CAIRO_OPERATOR_SOURCE operator
This avoids:
- the spill out effect over the property dialog
- unexpected color blending effects dur to the defaut CAIRO_OPERATOR_OVER operator