actor: Add CLUTTER_REQUEST_CONTENT_SIZE mode

Some actors want to have a preferred size driven by their content, not
by their children or by their fixed size.

In order to achieve that, we can extend the ClutterRequestMode
enumeration so that clutter_actor_get_preferred_size() defers to the
ClutterContent's own preferred size.

https://bugzilla.gnome.org/show_bug.cgi?id=676326
actor: Bail when setting the same content

No need to do all the work, if the content instance is the same.
actor: Reset the content box when setting a new content

We want to recompute the content box when changing the content instance,
in case the preferred size is different and the content gravity uses the
preferred size; the change of content with different preferred size and
same gravity should also trigger an implicit transition.

https://bugzilla.gnome.org/show_bug.cgi?id=711182
docs: Add missing deprecation annotations

https://bugzilla.gnome.org/show_bug.cgi?id=709252
conform: Drop a deprecated property

Use the non-deprecated :orientation property, instead of the
deprecated :vertical one.
script: Do not overwrite ObjectInfo fields

When merging multiple definitions it's possible that the ObjectInfo
fields may get overwritten. Instead of trampling over the fields, we
should reset them only when they actually change — especially the
"is_actor" one, which controls the destruction of the objects when
unmerging happens.

https://bugzilla.gnome.org/show_bug.cgi?id=669743
clutter/osx: add clutter_osx_disable_event_retrieval

https://bugzilla.gnome.org/show_bug.cgi?id=719962
osx: Clean up installed clutter-osx.h header

The installed header should not have private API declarations and
macros. Let's move those into the uninstalled clutter-backend-osx.h
header file instead.
docs: Add missing symbols
Document CLUTTER_BACKEND and CLUTTER_INPUT_BACKEND envvars

The allowed values are determined at configure time, but we can list all
the possible values, and assume people will actually check.

https://bugzilla.gnome.org/show_bug.cgi?id=681300