+ * The tree give the localisation of each object :
+ * - tree's root is in the background while tree's leafe are foreground objects
+ * - position of a node/Area depend on the layout : parent's position (posx, posy), size of previous brothers and parent's padding
+ * - size of a node/Area depend on the content (SIZE_BY_CONTENT objects) or on the layout (SIZE_BY_LAYOUT objects)
+ *
+ * DRAWING AND LAYERING ENGINE :
+ * Redrawing an object (like the clock) could come from an 'external event' (date change)
+ * or from a 'layering event' (position change).
+ * The following 'drawing engine' take care of all 'layering event'.
+ * 1) browse tree SIZE_BY_CONTENT
+ * - resize SIZE_BY_CONTENT node : children are resized before parent
+ * - if 'size' changed then 'resize = 1' on the parent
+ * 2) browse tree SIZE_BY_LAYOUT and POSITION
+ * - resize SIZE_BY_LAYOUT node : parent is resized before children
+ * - if 'size' changed then 'resize = 1' on childs with SIZE_BY_LAYOUT
+ * - calculate position (posx,posy) : parent is calculated before children
+ * - if 'position' changed then 'redraw = 1'
+ * 3) browse tree REDRAW
+ * - redraw needed objects : parent is drawn before children
+ *
+ * CONFIGURE PANEL'S LAYOUT :