[MENUBAR] Resolve the hit-item artifact bug (and the ugly workaround).
[MENUBAR] Fix hook proc: if 'code < 0', we must not handle the message.

According to the MSDN, code < 0 case must be only propgated to the
CallNextHookEx() without any further processing.
[MENUBAR] Fix few typos in comments.
[MENUBAR] Improve handling of focus and <F10> key.
[MENUBAR] Improve painting on Win XP and unthemed Vista and newer.