fix(grids): set z-index of row drag ghost to 1, #5748

This will make the z-index of the row drag ghost equal to the one of the grid.
This will not fix application scenario where grid is put in stacking container
with higher z-index. To fix this drag and drop directive should be able to use
outlet where drag ghost should be put in the DOM tree instead as a child
of the body.

Author: wnvko

projects/igniteui-angular/src/lib/core/styles/components/grid/_grid-theme.scss

@@ -886,6 +886,7 @@
 
         &%igx-grid__tr--ghost {
             background: --var($theme, 'row-ghost-background');
+            z-index: 1;
         }
     }