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.
Merge pull request #40 from elmart/master

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