sets an area where a menu can appear
| x1 | number | the leftmost coordinate by X-axis | 
| x2 | number | the rightmost coordinate by X-axis | 
| y1 | number | the topmost coordinate by Y-axis | 
| y2 | number | the bottommost coordinate by Y-axis | 
myMenu.setVisibleArea(x1, x2, y1, y2);
If the area isn't set, the menu will occupy all the available visible space.