[lmi] Popup contextual help appears at mouse position

[Greg Chicares]

http://msdn2.microsoft.com/en-us/library/ms971323.aspx
| SHIFT+F1
| Display tips help (context-sensitive help) near the selected control.

Instead, wxmsw displays popup help at the mouse position.

If the mouse position is far enough to the right or bottom, then
the help popup is truncated.

If the mouse is positioned over the msw taskbar, then no help
popup appears at all with msw "xp", at least not the way I've
got it configured (which isn't exotic AFAICT).

I think the positioning suggested by msdn is better anyway.