Grids; Row selector checkboxes are now square to bring in line with HTML Widget grids

Non HTML Widget grids, e.g., the Client, Contacts grid, have row selector checkboxes.

These checkboxes have been updated to look similar to those on HTML Widget grids (e.g., Task Manager folders), i.e., they are slightly larger and square rather than rounded.

Also, where the row selector displayed an icon (e.g., the Client, Contacts grid), the spacing has been improved so that the icon is no longer partially obscured by the checkbox.