[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/LineView.vue

@@ -66,7 +66,8 @@
                 y: 10,
                 x2: 10,
                 y2: 5,
-                stroke: '#717171'
+                stroke: '#717171',
+                useGrid: true
             };
         },
         inject: ['openmct'],