We need to call waitpid until no more waitable children are available.
This is necessary because SIGCHLD signals may coalesce into one when
multiple children terminate almost simultaneously.
Also, switch to EventLoop's asynchronous signal handling mechanism,
which allows more complex operations in the signal handler.
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.
During app teardown, the Application object may be destroyed before
something else, and so having Application::the() return a reference was
obscuring the truth about its lifetime.
This patch makes the API more honest by returning a pointer. While
this makes call sites look a bit more sketchy, do note that the global
Application pointer only becomes null during app teardown.
