cleanup

- move some methods to ResizeItem subclass
- allow distributor to instanciate sizer and resizeitem it needs
  through static factory methods, instead of passing in another ctor
  a distributor can only function with the right item and sizer anyways.
- use consistent import/export style
- remove obsolete code

src/components/views/rooms/RoomList.js

@@ -167,7 +167,7 @@ module.exports = React.createClass({
         const cfg = {
             onResized: this._onSubListResize,
         };
-        this.resizer = new Resizer(this.resizeContainer, RoomDistributor, cfg, RoomSizer);
+        this.resizer = new Resizer(this.resizeContainer, RoomDistributor, cfg);
         this.resizer.setClassNames({
             handle: "mx_ResizeHandle",
             vertical: "mx_ResizeHandle_vertical",