Add datepicker to timeline.

src/scss/date-picker.scss

@@ -0,0 +1,7 @@
+// The lit-flatpickr element itself is hidden as we only want the popup
+// calender.
+lit-flatpickr {
+  visibility: hidden;
+  width: 0px;
+  height: 0px;
+}

src/scss/timeline-core.scss

@@ -141,11 +141,14 @@ div.vis-tooltip {
   pointer-events: none;
 }
 
-ha-icon.lock {
+.timeline-tools {
   position: absolute;
   right: 2px;
   bottom: 2px;
   color: var(--primary-color);
   z-index: 10;
+}
+
+.timeline-tools ha-icon {
   cursor: pointer;
 }