Remove use of 'register' from Qt.

It is deprecated and clang is warning about it [-Wdeprecated-register].

Original patch from Stephen Kelly,
see commit d9fb6e6dbb2b322556d581265da2442e3b91a6a3 in qt5/qtbase

This is a backport of this commit + additional manual replacements in
header files. There are still some 'register' uses inside implementation files left,
but headers are all 'register'-free now.

Change-Id: I7225381df1f093073583d62fa86f7bd7cca869c7
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>