FrameSheet model object migration

Migration to a Domain object (currently a FrameSheetModel, feel free to
change its name). The model is being used by the slideshow (drawing
each tiles), animation preview (drawing animation) and drawing (update
model and redraw current tile).
Now the rendering information are not stored in a canvas element that
you paste from canvas to canvas but centralize in this model. The frame
is described as an array of array: that will allow different rendering
using the dpi constants and more flexibility (e.g. drawing a grid,
serializing the data).

Some minor modifications:
  - cleaning markup
  - adding background image to highlight transparent area

css/style.css

@@ -76,13 +76,19 @@ ul, li {
   list-style-type: none;
 }
 
-#preview-list .preview-tile {
+.preview-tile {
   padding : 10px;
   overflow: hidden;
+  background-color: gray;
+}
+
+.preview-tile .canvas-container {
+  float: left;
 }
 
 .preview-tile .tile-view {
   float: left;
+  border: blue 1px solid;
 }
 
 .preview-tile .tile-action {
@@ -99,9 +105,29 @@ ul, li {
 }
 
 #preview-list .preview-tile.selected {
-  background-color: gray;
+  background-color: lightyellow;
 }
 
+.canvas-container {
+  position: relative;
+  display: block;
+}
+
+.canvas-container .canvas-background {
+  background: url(../img/transparent_background.png) repeat;
+  position: absolute;
+  top: 0;
+  right: 0;
+  bottom: 0;
+  left: 0;
+}
+
+.canvas {
+  position: relative;
+  z-index: 1;
+}
+
+/* Force apparition of scrollbars on leopard */
 ::-webkit-scrollbar {
     -webkit-appearance: none;
     width: 7px;