seat: use focus_change event to update focused/active view

seat: use focus_change event to update focused/active view

- Connect to wlr_seat_keyboard_state's focus_change event.
- Add view_from_wlr_surface(), which does what the name says.
- Use focus_change event along with view_from_wlr_surface() to update
  server->focused_view and set SSD states via view_set_activated().
- Eliminate desktop_focused_view() since server->focused_view should be
  reliably up-to-date now.
- Eliminate view_focus/defocus() since we can now just call
  seat_focus_surface() directly.