Implement auto-completion support for wxTextEntry in wxOSX/Cocoa.

Implement auto-completion support for wxTextEntry in wxOSX/Cocoa.

Both completing a set of fixed strings and dynamic completion using a custom
completer are supported, although completing the file names remains MSW-only
for now.

Note that, unlike under MSW, auto-completion under Mac is not automatic and
has to be triggered manually by calling complete: method. This is done by
pressing F5 key by default. In the future we should call it automatically on a
timer event to make it more obviously discoverable.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67526 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775