Core: Renamed Symbol and PlacedSymbol classes and properties.

- Symbol -> SymbolDefinition
- PlacedSymbol -> SymbolItem
- Symbol#definition -> SymbolDefinition#item
- PlacedSymbol#symbol -> SymbolItem#definition
- Deprecate Project#symbols

Closes #770

examples/JSON/Gradients.html

@@ -17,7 +17,7 @@
             destination: path.bounds.rightCenter
         }
 
-        var symbol = new Symbol(path);
+        var symbol = new SymbolDefinition(path);
         symbol.place(view.center);
 
         window._json = project.exportJSON();

examples/JSON/Symbols.html

@@ -11,7 +11,7 @@
             to: [200, 100],
             fillColor: 'red'
         });
-        var symbol = new Symbol(ellipse);
+        var symbol = new SymbolDefinition(ellipse);
         var p1 = symbol.place([100, 100]);
         p1.rotate(45);
         var p2 = symbol.place([300, 200]);