title: Blockly: How to obtain value of a Dropdown or Checkbox Block
body: <p>I´m new to Blockly and can not find a way to obtain field value of a dropdown or checkbox.</p>
  
  <p>Lets consider following scenario (generated with blockly-dev-tools):</p>
  
  <pre><code>  Blockly.Blocks['feature'] = {
  init: function () {
    this.appendDummyInput()
      .appendField("Feature") // just for label
      .appendField(new Blockly.FieldDropdown([["manufacturer", "feature_manufacturer"], ["profile", "feature_profile"], ["glas", "feature_glas"]]), "category"); // for dropdown values
    this.appendValueInput("feature_name")
      .setCheck("String")
      .setAlign(Blockly.ALIGN_RIGHT)
      .appendField("Name");
    this.appendValueInput("feature_prio")
      .setCheck("Number")
      .setAlign(Blockly.ALIGN_RIGHT)
      .appendField("Priorität");
    this.appendDummyInput()
      .setAlign(Blockly.ALIGN_RIGHT)
      .appendField("Versteckt")
      .appendField(new Blockly.FieldCheckbox("TRUE"), "hidden");
  </code></pre>
  
  <p>now obtaining values from value inputs is not a problem, you can get thouse like this:</p>
  
  <pre><code>const featureName = element.getInputTargetBlock("feature_name");
        if (featureName) {
          console.log(featureName.getFieldValue("TEXT"));
        }
  const featurePrio = element.getInputTargetBlock("feature_prio");
        if (featurePrio) {
          console.log(featurePrio.getFieldValue("NUM"));
        }
  </code></pre>
  
  <p>but dummy inputs hosting dropdowns or checkboxes have no methods to provide selected values.
  It might be that this is my conceptual error to use dummy inputs to host the elements, but when using value input, you have always those nipples to the right, which are obsolate, since the values are provided by the checkbox or dropdown.</p>