Ensure that Enter key presses are never stolen from wxButton in wxMSW.

Ensure that Enter key presses are never stolen from wxButton in wxMSW.

This commit fixes the following bug: when an in-place editor control containing
an embedded button was used in wxDataViewCtrl, pressing Enter on the button
would close the editor, accepting changes, instead as (generic) wxDataViewCtrl
intercepts WXK_RETURN in its EVT_CHAR_HOOK handler. To prevent this from
happening, wxButton now handles EVT_CHAR_HOOK itself and never lets the parent
window intercept it if it's for WXK_RETURN. To ensure that normal
wxEVT_KEY_DOWN and wxEVT_CHAR are still generated in this case, wxButton
handler calls the new wxKeyEvent::DoAllowNextEvent() method that was added to
allow suppressing EVT_CHAR_HOOK only, without affecting the subsequent events.
DoAllowNextEvent() is currently only used in wxMSW but support for it was also
added to wxGTK and (both) wxOSX ports.

See #9102.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69984 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775