Make topics selectable

app/src/TopicViewModel.ts

@@ -0,0 +1,19 @@
+import { EventDispatcher } from '../../events'
+
+export class TopicViewModel {
+  private selected: boolean
+  public change = new EventDispatcher<void, TopicViewModel>(this)
+
+  public constructor() {
+    this.selected = false
+  }
+
+  public isSelected() {
+    return this.selected
+  }
+
+  public setSelected(selected: boolean) {
+    this.selected = selected
+    this.change.dispatch()
+  }
+}