dash: Don't notify two elements for each icon

dash: Don't notify two elements for each icon

When the user uses a screen reader and navigates through the
dash, each element is notified as two: one panel with no label,
and then a button with the application name as the label.

This is because each icon has the BUTTON role, but is inside a
container with the PANEL role, so each time the focus changes
from an icon to the next, there is a change in the container,
so the screen reader notifies this. But that container is
useless for the screen reader. The container always read its
label because the label_actor property is set.

This patch fixes this.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4182>