desktop-shell: Track focused shell surface by main surface

The focused surface is used for determining whether shell surfaces
are activated. They should also be considered activated when a
subsurface has focus. Inserting a call to
weston_surface_get_main_surface fixes this.

seat->focused_surface is only used for shell_surface keyboard focus
tracking.

Signed-off-by: Philipp Kerling <pkerling@casix.org>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>