Have the paint-editor container listen to selectedItems

This solves the issue of buttons not updating their disabled/enabled state until being dragged by making sure that a re-render occurs whenever a new item is selected, checking again for state.

src/containers/paint-editor.jsx

@@ -123,6 +123,7 @@ PaintEditor.propTypes = {
 };
 
 const mapStateToProps = state => ({
+    selectedItems: state.scratchPaint.selectedItems,
     undoState: state.scratchPaint.undo
 });
 const mapDispatchToProps = dispatch => ({