Radial Blur Preview & support for pointing center by clicking Doc

Concerning Filters / Blur / Radial Blur

1. Add Preview checkbox. If I remember correctly, in the 1990s it took a while for Radial Blur to render, so it is understandable how Preview was left out. Performance is no longer an issue.

2. Support for pointing the center position by clicking the desired center point on the Document while Radial Blur box is open in the foreground, in addition to the "blur center" box.