Detection improvement: Find visible descendant of non-visible clickable.

### Problem
Before, for a detected clickable element without visible
          client rect, an attempt was made to see if a floated direct
          child was visible. But this still missed some other cases
          (when child is not direct but descendant, or when child is
          not floated but positioned otherwise).
### Solution
Now, detection recurses down until finding a visible client
          rect, no matter how deep in the hierarchy or positioning
          method used.

### Notes
This fixes many non-detected items, at least in some pages I usually visit, so I think it's really worth it.
It could be thought that recursing down is too expensive. But try it. Works like a charm. No speed difference noticed by user. Your browser regularly does much more computing to show a regular webpage.