Only hide the browser selection once edit-mode is enabled

The recent shadow cursor implementation hides the native browser selection
highlights. However, the shadow cursor itself only starts being drawn
after edit-mode is enabled, meaning for a readonly document the selection
is invisible (#119)

This is a simple workaround that allows the native selection to be drawn
in circumstances where the shadow cursor is not active. Long term, the
shadow cursor should be also used on non-editable documents as detailed
in solution ideas for #119.
Clean-up the selection view when removing it

When a cursor is removed from the document (e.g., exiting edit mode),
the most recent selection is left visible.

* Don't re-append the overlayBottom on destroy
Upgrade from magical empty-string to const for shadow cursor memberId

In various parts of the code, the magical empty-string cursor id is used
to determine whether the object is a shadow cursor or not. Converting
this into a constant instead and removing the constructor arg prevents
some mistakes being made with this.