Fix generation of wxEVT_CHAR in wxMSW wxComboBox.

Fix generation of wxEVT_CHAR in wxMSW wxComboBox.

wxEVT_CHAR shouldn't be generated at all if wxEVT_KEY_DOWN was handled but it
still was for wxComboBox because the code in its MSW implementation directly
called HandleKeyDown() and HandleChar() methods, bypassing the logic dealing
with m_lastKeyDownProcessed at wxWindow level.

Fix this by calling MSWHandleMessage() instead to ensure that WM_CHAR after a
handled WM_KEYDOWN are ignored as they ought to.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71883 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775