Add .type metadata to preferences table (#701)

Allows shuffleboard to automatically discover the type, instead of inflexibly hardcoding it

wpilibj/src/main/java/edu/wpi/first/wpilibj/Preferences.java

@@ -63,6 +63,7 @@ public class Preferences {
    */
   private Preferences() {
     m_table = NetworkTableInstance.getDefault().getTable(TABLE_NAME);
+    m_table.getEntry(".type").setString("RobotPreferences");
     // Listener to set all Preferences values to persistent
     // (for backwards compatibility with old dashboards).
     m_table.addEntryListener(