Fix usage of binding values to datepickers

packages/bbui/src/Form/DatePicker.svelte

@@ -1,6 +1,7 @@
 <script>
   import Field from "./Field.svelte"
   import DatePicker from "./Core/DatePicker.svelte"
+  import { createEventDispatcher } from "svelte"
 
   export let value = null
   export let label = null
@@ -10,6 +11,13 @@
   export let enableTime = true
   export let placeholder = null
   export let appendTo = undefined
+
+  const dispatch = createEventDispatcher()
+
+  const onChange = e => {
+    value = e.detail
+    dispatch("change", e.detail)
+  }
 </script>
 
 <Field {label} {labelPosition} {error}>
@@ -20,6 +28,6 @@
     {placeholder}
     {enableTime}
     {appendTo}
-    on:change
+    on:change={onChange}
   />
 </Field>