I've added a preview for emboss, motion blur and twirl : while the dialog is shown, the image is updated, like with colorize and intensity dialogs.
I'm not sure how to do filter preview. Maybe with a thread.
EDIT: I've uploaded last changes as version 4.2 as a zip file. It's essentially to make source code available, because there are few changes from the user side.
Now, if someone make a new tool, it can be easily merged into the project. To do so, just create a unit as explained above (TGenericTool class and RegisterTool call).