[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/BoxView.vue

@@ -54,7 +54,8 @@
                 x: 1,
                 y: 1,
                 width: 10, 
-                height: 5 
+                height: 5,
+                useGrid: true
             };
         },
         inject: ['openmct'],