Set shape background to be transparent by default (#330)

Also makes "Clear canvas" reset the entire app state

src/appState.ts

@@ -0,0 +1,22 @@
+import { AppState } from "./types";
+import { getDateTime } from "./utils";
+
+const DEFAULT_PROJECT_NAME = `excalidraw-${getDateTime()}`;
+
+export function getDefaultAppState(): AppState {
+  return {
+    draggingElement: null,
+    resizingElement: null,
+    elementType: "selection",
+    exportBackground: true,
+    currentItemStrokeColor: "#000000",
+    currentItemBackgroundColor: "transparent",
+    currentItemFont: "20px Virgil",
+    viewBackgroundColor: "#ffffff",
+    scrollX: 0,
+    scrollY: 0,
+    cursorX: 0,
+    cursorY: 0,
+    name: DEFAULT_PROJECT_NAME
+  };
+}