Generate the same flags for modifier key events in wxGTK as in wxMSW.

Generate the same flags for modifier key events in wxGTK as in wxMSW.

Resolve the discrepancy between modifier flags of the key events corresponding
to the modifier key itself between wxMSW and wxGTK by changing the latter to
follow wxMSW behaviour.

Clearly document the now officially correct behaviour of the modifiers for the
key events corresponding to the modifiers keys themselves in the manual.

This fix also makes it unnecessary to work around this bug in the keyboard
unit test so remove it from there.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65527 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775