Watermark tool usability improvements

[XnSoldier]

I am talking about the watermark tool when a picture is opened, not the function in batch processing

- please use last watermark by default
I think most people apply the same watermark repeatedly. But the tool always asks for a watermark file. and any resizing are reset.

- add a size option with % in size or pixel width

- possibly add "recent watermarks" to make it easier when a few watermarks get used repeatedly. Add a "clear" list just in case paths/watermarks or missing.

- add keyboard shortcut (I do I miss anything) to apply watermark.

- give warning when saving picture but watermark is not applied. Why you see (watermark on picture) is not being saved if not applied.

thank you for such a great tool with XnView MP!!! Love the speed and quality renderings for batch processing.