Set initial wxTextCtrl text earlier in wxOSX

This reverts the changes of 63bcc669d8 (fixing setting initial value
under osx_cocoa for single line text controls, 2009-10-01) and fixes the
problem which this commit probably tried to fix in a different way,
using the same approach as in 98f5315405 (Don't set initial label in
wxNativeWindow on OS X, 2016-04-29) as the real root of the problem was
that the text set in CreateTextControl() was overwritten later when the
label was set from SetPeer().

This change means that the text is now set correctly before SetPeer()
calls SetInitialSize() call, which makes it possible to set the correct
initial size based on the initial text, as will be done in later
commits.

It also makes Cocoa port more consistent with iOS one, as a nice side
effect.

include/wx/osx/cocoa/private/textimpl.h

@@ -32,6 +32,12 @@ public :
     virtual ~wxNSTextBase() { }
 
     virtual bool ShouldHandleKeyNavigation(const wxKeyEvent &event) const wxOVERRIDE;
+
+    virtual void SetInitialLabel(const wxString& WXUNUSED(title), wxFontEncoding WXUNUSED(encoding)) wxOVERRIDE
+    {
+        // Don't do anything here, text controls don't have any label and
+        // setting it would overwrite the string value set when creating it.
+    }
 };
 
 // implementation exposed, so that search control can pull it