shell: restrict panel centering to specific cases

commit 02302c9b implemented vertical centering of the panel being activated, but we should restrict that to only happen when the activation is from the Search view or from the set_active_panel_from_id() CcShell iface, which covers the case of panel activated from commandline parameter or from other panels.

The above does not include activating panels manually by mouse or keyboard which is the problematic case explained in issue #2506 (closed)

Closes #2506 (closed)