Move platform-specific functionality into Platform

Platform classes are provided by the application via
PlatformPeg.set().

src/PlatformPeg.js

@@ -0,0 +1,50 @@
+/*
+Copyright 2016 OpenMarket Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/*
+ * Holds the current Platform object used by the code to do anything
+ * specific to the platform we're running on (eg. web, electron)
+ * Platforms are provided by the app layer.
+ * This allows the app layer to set a Platform without necessarily
+ * having to have a MatrixChat object
+ */
+class PlatformPeg {
+    constructor() {
+        this.platform = null;
+    }
+
+    /**
+     * Returns the current Platform object fir the application.
+     * This should be an instance of a class extending BasePlatform.
+     */
+    get() {
+        return this.platform;
+    }
+
+    /**
+     * Sets the current platform handler object to use for the
+     * application.
+     * This should be an instance of a class extending BasePlatform.
+     */
+    set(plaf) {
+        this.platform = plaf;
+    }
+}
+
+if (!global.mxPlatformPeg) {
+    global.mxPlatformPeg = new PlatformPeg();
+}
+module.exports = global.mxPlatformPeg;