title: Problems with state with Vue component and TypeScript
body: <p>I have a problem with the following simple Vue component. I'm just trying to get the v-if="isTouched" to rerender upon setting the setter (by firing the touch event). 
  According to Vue dev tools the _isTouched variable is "undefined". From what I've understood variables like that shall automatically be included in the components data() block. This however doesnt seem to be the case. 
  Any ideas on what I'm missing here?</p>
  
  <p>Best//M</p>
  
  <pre><code>&lt;template&gt;
      &lt;div v-if="selectedProgramItem"&gt;
          {{isTouched}}
          &lt;span v-if="isTouched"&gt;Unsaved changes&lt;/span&gt;
          &lt;input type="text" class="form-control" v-model="selectedProgramItem.title" /&gt;
          &lt;button @click="touch()"&gt;Touch object&lt;/button&gt;
      &lt;/div&gt;
  
  &lt;/template&gt;
  &lt;script lang="ts"&gt;
  import { Component, Prop, Vue } from 'vue-property-decorator';
  import { ProgramItem } from '@/entities/ProgramItem';
  
  @Component
  export default class ProgramEditItemPane extends Vue {
      @Prop() private selectedProgramItem?: ProgramItem;
  
      _isTouched: boolean = false;
  
      private get isTouched(): boolean {
              return this._isTouched;
      }
      private set isTouched(val: boolean) {
          this._isTouched = val;
          // this._isTouched &lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt; == undefined here.
      }
      private touch() {
          this.isTouched = true;
      }
  
  }
  &lt;/script&gt;
  </code></pre>