Fix int field of wxCommandEvents generated by menu items in wxMSW.

Fix int field of wxCommandEvents generated by menu items in wxMSW.

Set the int field of wxCommandEvent generated by clicking on the menu items
correctly for not checkable items: it is supposed to be -1 and not 0 (which is
the value for checkable but currently unchecked items). This was already the
case for wxGTK and wxOSX and implied by the comments in the code.

Make wxMSW behave like this too and clearly document this behaviour.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69101 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775