Fix "Enter" behaviour when wxEVT_TEXT_ENTER is skipped in wxGTK

Bring wxGTK in sync with wxMSW and wxMac by activating the default
button manually if wxEVT_TEXT_ENTER handler skips the event.

include/wx/gtk/textentry.h

@@ -86,6 +86,10 @@ protected:
 
     static int GTKGetEntryTextLength(GtkEntry* entry);
 
+    // Helper for wxTE_PROCESS_ENTER handling: activates the default button in
+    // the dialog containing this control if any.
+    bool ClickDefaultButtonIfPossible();
+
 private:
     // implement this to return the associated GtkEntry or another widget
     // implementing GtkEditable