title: Adding TornadoFX object removes other elements of stage
body: <p>I was playing around with TornadoFX and wanted to add a horizontal line to my screen, to see how it works. I added it to my code as follows:</p>
  <pre class="lang-kotlin prettyprint-override"><code>    private val menuView: MenuView by inject()
      private val controller: MainController by inject()
  
      override val root = borderpane {
  
          top = menuView.root
  
          style {
              backgroundColor += Color.WHITE
          }
  
          val data = controller.getData()
  
  
          center {
  
              for (i in 0 until data.count()) {
                  val values = data[i]
                  datagrid(values) {
                      if (data.count() &gt; 0) {
                          cellWidth = (8.0 * (values.maxBy { it.root.count() }!!.root.count()))
                      }
                      cellHeight = 20.0
  
                      horizontalCellSpacing = 0.0
  
                      maxCellsInRow = controller.maxNum
                  }
              }
              
              line {
                  startY = 3000.0
                  endY = 3000.0
                  startX = 500.0
                  endX = 5000.0
              }
  
          }
      }
  </code></pre>
  <p>It seems that adding the line inside the <code>center</code> component leads to it being the only thing rendered. The desired result is achieved by replacing the <code>borderpane</code> with a <code>stackpane</code>, as follows:</p>
  <pre class="lang-kotlin prettyprint-override"><code>
      private val menuView: MenuView by inject()
      private val controller: MainController by inject()
  
      override val root = stackpane {
  
          style {
              backgroundColor += Color.WHITE
          }
  
          val data = controller.getData()
  
  
          for (i in 0 until data.count()) {
              val values = data[i]
              datagrid(values) {
                  if (data.count() &gt; 0) {
                      cellWidth = (8.0 * (values.maxBy { it.root.count() }!!.root.count()))
                  }
                  cellHeight = 20.0
  
                  horizontalCellSpacing = 0.0
  
                  maxCellsInRow = controller.maxNum
              }
          }
  
          line {
              startY = 3000.0
              endY = 3000.0
              startX = 500.0
              endX = 5000.0
          }
      }
  
  </code></pre>
  <p>However, this removes the menu bar from the top, which I also wanted to keep. Is there a way to have both?</p>