Always call superclass when focusing Settings View

Previously, in some cases, calling focus wasn't causing the
selected panel to correctly focus if the Settings View was
focused by cycling through tabs with a keyboard shortcut,
as described in atom/atom#2416.

This seems to happen only for panels which don't implement
their own focus handler, e.g. the Settings panel and panels
for specific packages. Panels which do have a focus handler
implemented weren't affected.

This change moves the focus() call to the superclass so that
it's always executed (allowing the panel to focus correctly
even if it doesn't have a focus handler), but also allows
the panel's focus handler to be executed afterwards if it
does exist.

Fixes atom/atom#2416