Qt should honor /usr/share/pixmaps as a valid icon directory on Linux

According to the Freedesktop.org icon theme specification (http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#directory_layout), applications should look in the /usr/share/pixmaps directory. From what I can tell, Qt doesn't look for icons in this directory.

I've attached a patch that adds this functionality. You could improve this patch by adding a helper function to generate the entries, as the code is largely the same regardless of whether the icon is loaded from a theme directory or from /usr/share/pixmaps.