Support a trackBy function for performance

[the-ult]

I'm submitting a ... (check one with "x")

[ ] bug report => search github for a similar issue or PR before submitting
[x ] feature request
[ ] support request => Please do not submit support request here

Current behavior
I'm currently using the ngx-dnd with some extra filters and data which is loaded / triggered by an updated [BehaviorSubject] (Which means a total repaint/rebuilt/re-rendering every time)

The items have a lot of info (html, data, etc)

<ngx-dnd-container  [model]="availableBookItems$ | async | filterBy: filter | orderBy: 'book.displayName '"
        [dropZones]="['restricted-target']">
        <md-accordion class="__playlistitems-container">
          <ng-template let-availableBookItem="model">
            <md-expansion-panel class="u-movable" hideToggle="true">
              <md-expansion-panel-header expandedHeight="88px" collapsedHeight="72px">
                <md-panel-title fxFlex="60">
                  <bsm-availablebook-panel-title [book]="availableBookItem.book"></bsm-availablebook-panel-title>
                </md-panel-title>
                <md-panel-description fxFlexLayout fxLayoutAlign="end center" class=" u-text-align-right">
                  <span *ngIf="availableBookItem.book.collectionName" class="__subheader">
                  {{ availableBookItem.book.collectionName }}
                </span>
                  <button md-icon-button color="primary" (click)="addToPlaylist(availableBookItem); removeFromAvailable(availableBookItem)"
                    mdTooltip="Voeg boek toe aan de afspeellijst">
                  <md-icon>playlist_add</md-icon>
                </button>
                </md-panel-description>
              </md-expansion-panel-header>
              <!-- Panel content -->
              <bsm-book-panel-content [book]="availableBookItem.book"></bsm-book-panel-content>
            </md-expansion-panel>
          </ng-template>
        </md-accordion>
      </ngx-dnd-container>

It would be great if we could provide a/the trackBy function, to improve the performance

Please tell us about your environment:

    _                      _                 ____ _     ___
   / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
  / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
 / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
/_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
               |___/
@angular/cli: 1.4.3
node: 8.1.4
os: win32 x64
@angular/animations: 4.4.3
@angular/cdk: 2.0.0-beta.10-1b6b270
@angular/common: 4.4.3
@angular/compiler: 4.4.3
@angular/core: 4.4.3
@angular/flex-layout: 2.0.0-rc.1
@angular/forms: 4.4.3
@angular/http: 4.4.3
@angular/material: 2.0.0-beta.10-1b6b270
@angular/platform-browser: 4.4.3
@angular/platform-browser-dynamic: 4.4.3
@angular/platform-server: 4.4.3
@angular/router: 4.4.3
@angular/cli: 1.4.3
@angular/compiler-cli: 4.4.3
@angular/language-service: 4.4.3
typescript: 2.5.2

• ngx-charts version: x.x.x

"@swimlane/ngx-dnd": "~3.0.0",