Use aria-activedescendant attribute to update node in ElectronShellView

[jbphet]

There is a TODO in ElectronShellView about using the aria-activedescendant attribute to update the active node without changing focus. To be perfectly honest, I don't fully understand what that means, but I'm in the process of making sure that all TODO items have associated GitHub issues (see phetsims/tasks#1017) so I'm adding this issue to make sure that the TODO is addressed at some point.

See phetsims/build-an-atom#194 and phetsims/scenery#873 for more information on this.

@jessegreenberg added the original TODO comment, so I'm assigning to him to address, prioritize, reassign, or defer at his discretion. @jessegreenberg - feel free to edit the title and/or description if it will help to make things more clear.