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/resizer/sizer.js

@@ -18,7 +18,7 @@ limitations under the License.
 implements DOM/CSS operations for resizing.
 The sizer determines what CSS mechanism is used for sizing items, like flexbox, ...
 */
-export class Sizer {
+export default class Sizer {
     constructor(container, vertical, reverse) {
         this.container = container;
         this.reverse = reverse;
@@ -86,10 +86,3 @@ export class Sizer {
         }
     }
 }
-
-export class FlexSizer extends Sizer {
-    setItemSize(item, size) {
-        item.style.flexGrow = `0`;
-        item.style.flexBasis = `${Math.round(size)}px`;
-    }
-}