Fixes a spelling mistake of a filename (readme)
tree keyboard handling: we never tested for ShiftState
popupmenu: Adds a convenience function AddSeparator()

I like less typing. ;-)
Menu shortcuts with F1 key combinations never worked.

The global F1 (application help) shortcut handling
never looked at the ShiftState value. So if we had
a popup menu with F1->help; Ctrl+F1->About, the
second item's shortcut with be interpreted as F1 only.