Don't release NSProgressIndicator too soon, we need to be able to keep
it to use it later, so move "release" from wxAppProgressDockIcon ctor to
its dtor.
OTOH, remove an extraneous "retain" to fix memory leak of
wxAppProgressDockIcon.
Finally, show the indicator, if it had been hidden, when Pulse() is
called for consistency with SetProgress().
Closes https://github.com/wxWidgets/wxWidgets/pull/1150
