Add a helper function to generate an unique id.

This function will be needed later when a widget without an id needs to
do some registration in new window functions.

src/gui/widgets/helper.cpp

@@ -170,5 +170,15 @@ t_string missing_widget(const std::string& id)
 	return t_string(vgettext("Mandatory widget '$id' hasn't been defined.", symbols));
 }
 
+std::string get_uid()
+{
+	static unsigned id = 0;
+	++id;
+	assert(id); // avoid wrapping.
+
+	return "____" + lexical_cast<std::string>(id);
+
+}
+
 } // namespace gui2
 

src/gui/widgets/helper.hpp

@@ -132,6 +132,16 @@ void restore_background(const surface& restorer,
  */
 t_string missing_widget(const std::string& id);
 
+/**
+ * Gets an unique id for a widget.
+ *
+ * The id will have extra leading underscores so it's in the private range and
+ * can't collide with user defined ids.
+ *
+ * @returns                       The id.
+ */
+std::string get_uid();
+
 } // namespace gui2
 
 #endif