Added private wxEVT_AFTER_CHAR event for wxMSW implementation needs.

Added private wxEVT_AFTER_CHAR event for wxMSW implementation needs.

This event is sent by wxMSW after the default handling of WM_CHAR has taken
place. It can be used to define an event handler triggered by key presses and
having access to the new value of the control, updated to take the last key
press into account.

This event will be used by auto-completion implementation for wxMSW only for
now.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67512 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775