Split wxTextCompleter into a base class and wxTextCompleterSimple.

Split wxTextCompleter into a base class and wxTextCompleterSimple.

Allow overriding either the iterator-like methods of the base class or the
single and possibly more convenient, albeit slightly less efficient, method of
the derived wxTextCompleterSimple class.

This makes it possible to completely delegate to wxTextCompleter from wxMSW
IEnumString implementation and actually makes the code there easier, even if
it it still not quite trivial because of multi-threading concerns.

It also would make it possible to show the completions progressively, as they
are retrieved, in a future generic implementation of auto-completion (MSW
native implementation doesn't do this unfortunately and waits until all of the
completions become available before showing them).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67515 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775