Refactor: extract wxTextCompleterFixed from wxMSW to a header.

This class will be used in other ports too so don't make it private to wxMSW
(although it still remains private to wxWidgets for now as it doesn't make
much sense to use it in user code).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67525 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775