Added ability to randomize playlists

Missing videos now show a more verbose error in the logs

src/app/dialogs/modify-playlist/modify-playlist.component.html

@@ -9,6 +9,10 @@
             </mat-form-field>
         </div>
 
+        <div>
+            <mat-checkbox [(ngModel)]="playlist.randomize_order"><ng-container i18n="Randomize order when playing checkbox label">Randomize order when playing</ng-container></mat-checkbox>
+        </div>
+
         <div style="margin-bottom: 10px; height: 40px;">
             <div style="float: left">
                 <span *ngIf="reverse_order === false" i18n="Normal order">Normal order&nbsp;</span>

src/app/dialogs/modify-playlist/modify-playlist.component.ts

@@ -85,6 +85,7 @@ export class ModifyPlaylistComponent implements OnInit {
       index = this.playlist_file_objs.length - 1 - index;
     }
     this.playlist_file_objs.splice(index, 1);
+    this.playlist.uids.splice(index, 1);
     this.processFiles();
   }