Fix setting colours for the edit control part of wxComboBox.

Fix setting colours for the edit control part of wxComboBox.

Recognize this control as part of wxComboBox and so handle WM_CTLCOLOR for it.

To do this, override ContainsHWND() in wxComboBox and use it, on the parent
window, in wxControl::DoMSWControlColor(), if we fail to find the window
directly.

Closes #811.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72468 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775