Fix WriteText() not scrolling to bottom with GTK >= 3.14

The introduction of scrollbar animations broke scroll-to-end functionality when
large amounts of text are written, due to interactions with GtkTextView's
background layout. Work around this by scrolling after the layout has finished.
See #18864

include/wx/gtk/textctrl.h

@@ -143,6 +143,7 @@ public:
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);
 
     void GTKOnTextChanged() wxOVERRIDE;
+    void GTKAfterLayout();
 
 protected:
     // overridden wxWindow virtual methods
@@ -216,8 +217,9 @@ private:
     // a dummy one when frozen
     GtkTextBuffer *m_buffer;
 
-    GtkTextMark* m_showPositionOnThaw;
+    GtkTextMark* m_showPositionDefer;
     GSList* m_anonymousMarkList;
+    unsigned m_afterLayoutId;
 
     // For wxTE_AUTO_URL
     void OnUrlMouseEvent(wxMouseEvent&);