shell: keyboard focus and restacking fixes for sub-surfaces

shell: keyboard focus and restacking fixes for sub-surfaces

The shell needs to redirect some actions to the parent surface, when
they originally target a sub-surface. This patch implements the
following:

- Move, resize, and rotate bindings always target the parent surface.

- Opacity (full-surface alpha) binding targets the parent surface. This
  is broken, because it should change the opacity of the whole compound
  window, which is difficult to implement in the renderer.

- click_to_activate_binding() needs to check the shell surface type from
  the main surface, because sub-surface would produce SHELL_SURFACE_NONE
  and prevent activation.

- Also activate() needs to check the type from the main surface, and
  restack the main surface. Keyboard focus is assigned to the original
  (sub-)surface.

- focus_state_surface_destroy() needs to handle sub-surfaces: only the
  main surface will be in a layer list. If the destroyed surface is
  indeed a sub-surface, activate the main surface next. This way a
  client that destroys a focused sub-surface still retains focus in the
  same window.

- The workspace_manager.move_surface request can accept also
  sub-surfaces, and it will move the corresponding main surface.

Changes in v2:
- do not special-case keyboard focus for sub-surfaces
- fix surface type checks for sub-surfaces in shell, fix restacking of
  sub-surfaces in shell, fix focus_state_surface_destroy()

Changes in v3:
- Renamed weston_surface_get_parent() to
  weston_surface_get_main_surface() to be more explicit that this is
  about sub-surfaces
- Fixed move_surface_to_workspace() to handle keyboard focus on a
  sub-surface.
- Used a temporary variable in several places to clarify code, instead
  of reassigning a variable.
- Fixed workspace_manager_move_surface() to deal with sub-surfaces.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>