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

interface/wx/textcompleter.h

@@ -74,6 +74,10 @@ public:
         otherwise they will be simply ignored, making adding them to the array
         in the first place useless.
 
+        Notice that this function may be called from thread other than main one
+        (this is currently always the case under MSW) so care should be taken
+        if it needs to access any shared data.
+
         @param prefix
             The possibly empty prefix that the user had already entered.
         @param res