switcherPopup: Implement functions to add and remove accessible states

switcherPopup: Implement functions to add and remove accessible states

Set the accessible states of the switcherList items by calling a
function instead of manipulating class-internal variables from outside
the class, which is considered bad practice.

The check whether the item at `_selectedIndex` exists can also be
removed since we always select a new index after an item was removed
(i.e. an app was closed) and destroy the alt-tab switcher right away if
no more items exist (see `SwitcherPopup._itemRemovedHandler`).

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/167