Make wxChoice and wxComboBox behaviour same as in native controls in wxMSW.

Make wxChoice and wxComboBox behaviour same as in native controls in wxMSW.

Keep the item selected from the drop down using keyboard when switching away
from the control by pressing TAB: although this generates CBN_SELENDCANCEL
notification, the selection is actually kept by the native controls in this
case, so don't reset it ourselves -- even though it makes sense, it makes wx
applications behave differently from the native ones.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73103 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775