fix problem with splash window on OS X: requesting a redraw of a hidden window doesn...

fix problem with splash window on OS X: requesting a redraw of a hidden window doesn't result in an expose event being delivered.

The splash window was being hidden by default on OS X as soon as someone called Splash::pop_back_for(), because of window layering issues on OS X. But then
the next call to ::message() would not return because a recursive event loop was started that waits for the expose event, which never comes. So, (a) show the
window when ::message() is called AND (b) remove the recursive event loop because (b.1) we don't seem to need it (b.2) recursive event loops are generally evil.