prepare for hexagon display & other models

- display cell as hexagon is Display.mHexagon if set ( not default )
will need a checkbox to set this
- various new models preparation...

java/org/artisanlogiciel/games/maze/model/HalfSquareProvider.java

@@ -0,0 +1,6 @@
+package org.artisanlogiciel.games.maze.model;
+
+public interface HalfSquareProvider {
+
+    long getLeftDown(int x, int y);
+}