ToolTipHandler.cs: fix issue #230 ("using tooltip.Visible property make tooltip paused/static but not hidden").
Cause: when it stop the periodic update it also skip updating Visible status for ToolTipForm (the renderer).
Fix: call the periodic update once when changing ToolTipHandler Visible status.

ToolTipForm.cs & ToolTipHandler.cs: some small cleanup aiming to increase code readability
Merge pull request #233 from xponen/zkl_issue230

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