OS X-style behavior for popups on all platforms

qapplication_mac.cpp implements behavior for popups that matches
standard OS X behavior: a click outside of an active popup will close
the popup, but will not do anything else. We would like this behavior in our application, but we are using Linux.

We'd like it if this behavior were not limited to the Mac build, but
were rather available on any platform and specified with a style hint
(not unlike "QStyle::SH_Menu_SloppySubMenus").