Application::show_tooltip() now keeps track of the application's active
tooltip source widget so it can be updated while being shown when the
same widget updates its tooltip label.
Application::hide_tooltip() will unset the tooltip source widget,
respectively.
This is pretty useful for the ResourceGraph applet's tooltips!
Also re-use the Application::TooltipWindow's rect position in its
set_tooltip() method to avoid flickering from the window temporarily
being moved to 100, 100 and the position adjusted moments later.