Fixed Layers in svgcanvas. Moved Layer class. New HistoryRecordingservice.

Canvas was referencing drawing.all_layers and drawing.current_layer.
Both variables now represent Layer instead of group element and should
be considered private.
Moved Layer class to layer.js
New HistoryRecordingService added to help with moving Layer code out of
Canvas. Started using it in Canvas.mergLayer

editor/svg-editor.html

@@ -39,10 +39,12 @@
   <script src="svgutils.js"></script>
   <script src="sanitize.js"></script>
   <script src="history.js"></script>
+  <script src="historyrecording.js"></script>
   <script src="coords.js"></script>
   <script src="recalculate.js"></script>
   <script src="select.js"></script>
   <script src="draw.js"></script>
+  <script src="layer.js"></script>
   <script src="path.js"></script>
   <script src="svgcanvas.js"></script>
   <script src="svg-editor.js"></script>