core: Remove view and viewport handling from CogShell

Remove handling of viewports and views from CogShell. Tracking live
viewports can be done using CogPlatform since #683 and platform
plug-ins no longer depend on CogShell for that. The following are
no longer needed in CogShell:

- The :web-view and :viewport properties, and their accessors.
- The ::create-view and ::create-viewport signals.
- The .create_view() virtual function and its default implementation.
- The cog_shell_add_viewport(), cog_shell_remove_viewport(),
  cog_shell_get_n_viewports(), and cog_shell_get_nth_viewport()
  methods.

As a side effect of the above removals, the examples have been changed
to have them create their views using cog_view_new() instead of relying
on the default implementation of the CogShell.create_view() virtual
function.

The launcher also needed a number of changes, which now manages itself
a viewport as it cannot rely on the shell for that. Some code was moved
around to accommodate creation of the view in cog_launcher_startup()
without relying on the CogShell::create-view signal, and similarly for
web view creation when running in automation mode.

There are more opportunities for further simplifications that would be
better done in separate patches: this one focuses on the minimum amount
of changes needed to get view and viewport handling out of CogShell.