* make CallTip and ToolTip sub-classes of a common abstract base class
* remove ListboxToolTip (unused and ugly)
* greatly increase test coverage
* tested on Windows, Linux and macOS
Fix-up class name duplication in PR #7807. Combined effect is that
module calltips and its class CallTips are now calltip and Calltip.
In module calltip_w class CallTip is now CalltipWindow.