ZKL: fix issue #230 ("using tooltip.Visible property make tooltip static but not hidden").

Cause: I made it (tooltip.Visible) stop periodic update, but this also skipped updating the renderer's Visible status.
Fix: so I make it call the periodic update at least once so it at least have 1 chance to change the renderer's Visible status.

Others:
ToolTipForm.cs & ToolTipHandler.cs: also some small cleanup to increase code readability