Change sorting function in order to add capability to sort both ways

layer-selector.h

@@ -30,6 +30,11 @@
 #include <glib.h>
 #include "mapping-parser.h"
 
+/**
+ * @brief Defines how to sort the layer selector list box.
+ */
+enum layer_selector_sort_algo {LAYER_SELECTOR_SORT_DOWN = 0, LAYER_SELECTOR_SORT_UP};
+
 /**
  * @brief Generate layer widgets in \p listbox
  * @note This clears all previously inserted elements