shell: update sidebar when active panel is set externally

When the active panel is not changed through sidebar navigation,
e.g. by calling the Control Center through command line and asking
to open a specific panel, the panel is correctly opened but the
sidebar is not updated to reflect that.

Fix that by selecting the externally set panel row, and eventually
moving to the correct list.

https://bugzilla.gnome.org/show_bug.cgi?id=769063