[Display Layout] Snap to grid toolbar toggle button (#2262)

* Add a  button in the toolbar to toggle snapp to grid.

* Convert item's coordinates and size from grid unit to pixels depending on useGrid value.

* Rename selectionListener to removeSelectionListener

src/plugins/displayLayout/components/ImageView.vue

@@ -56,7 +56,8 @@
                 y: 1,
                 width: 10,
                 height: 5,
-                url: element.url
+                url: element.url,
+                useGrid: true
             };
         },
         inject: ['openmct'],