[Display Layout] Add delete button (#2251)

* Add delete button in the toolbar for removing items

* Mutate composition if there are no telemetry objects. Select the parent layout after deleting an item.

* Saving work

* Watch for index in the components and update it in the context.

* Select the parent after a composition is removed.

* Address reviewer's feedback.
- Rename mutatComposition() to removeFromComposition().
- Inline logic for filtering composition
- Use separate branches for each item type in trackItem().

* Address reviewer's requested changes

src/plugins/displayLayout/components/BoxView.vue

@@ -75,16 +75,22 @@
                 };
             }
         },
+        watch: {
+            index(newIndex) {
+                if (!this.context) {
+                    return;
+                }
+
+                this.context.index = newIndex;
+            }
+        },
         mounted() {
-            let context = {
+            this.context = {
                 layoutItem: this.item,
                 index: this.index
             };
             this.removeSelectable = this.openmct.selection.selectable(
-                this.$el,
-                context,
-                this.initSelect
-            );
+                this.$el, this.context, this.initSelect);
         },
         destroyed() {
             if (this.removeSelectable) {