Set wxKeyEvent::m_rawFlags to hardware key code in wxGTK.

Set wxKeyEvent::m_rawFlags to hardware key code in wxGTK.

The raw flags were previously unused in wxGTK but hardware key code is an
important information which may be useful to the application, so pass it in
the flags (this is rather symmetric with passing lParam in it under MSW as
lParam contains the scan code, among other things).

Also document the meaning of raw key code and flags in all the major ports.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65893 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775