Allow setting hints for multi-line wxTextCtrl when supported.

Don't prevent people from using hints in wxMSW and wxGTK2, where they work
with multiline text controls too, even though they do not work with wxGTK3 nor
wxOSX.

Closes #14456.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78316 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/textctrl.h

@@ -159,6 +159,7 @@ protected:
     virtual bool UseGTKStyleBase() const wxOVERRIDE { return true; }
 
     virtual void DoSetValue(const wxString &value, int flags = 0) wxOVERRIDE;
+    virtual wxString DoGetValue() const wxOVERRIDE;
 
     // Override this to use either GtkEntry or GtkTextView IME depending on the
     // kind of control we are.