Don't block the main UI thread while generating completions in wxMSW.

Don't block the main UI thread while generating completions in wxMSW.

The native IAutoComplete implementation takes care to retrieve the completions
from a background thread to prevent the UI from freezing while they're being
generated, but we worked against it by always generating all the completions
from the main thread and just enumerating them from the background one.

Change this now and call wxTextCompleter::GetCompletions() method from the
background thread itself to never block the main one.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67514 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775