Opal::CallManager: Fixed maximum frame height.

Using engine constants allows preventing errors.
Opal::CallManager: Allow a bigger bitrate.

Also changed the signification of the maximum rx bitrate. This is not
really the allowed reception bitrate. This is actually the allowed
bitrate (rx & tx) for the Ekiga endpoint.
Opal::CallManager: Do not hardcode the video clock rate.
Opal::CallManager: Removed another hardcoded video clock rate.
CodecDescription: Large rework of CodecDescription and CodecList.

The general idea behind this change is to move codec list loading from
the configuration where it belongs: in the child Opal class and not in
the main base class. This allows more control over what we are doing and
different CallManager could have different codec handling mechanisms.
The newly introduced load method allows loading codecs descriptions from
the configuration is dependant on the child implementation. The Opal
component uses its own "load" method.

The CodecList class has been simplified. Instead of handling an internal
list, it inherits of a std::list of std::string, where std::string is
the internal representation of the enabled/disabled codec.

The codecs blacklist is now handled in the Opal CodecList
implementation. This is cleaner.

Furthermore, we do not rely on the SDP encoding name for the codec. This
allows the user to change the priority between several variations of the
same codecs, e.g. H.264 High-Profile and H.264.

Finally, a known-codecs.h file has been added. Even though codecs are
plugins, it allows giving more useful information to the user.
CodecsBox: Ported to the new CodecList/CodecDescription API.
CallCore: Added methods to get and set codecs.

This will use the different CallManagers to do so.

It allows the CodecsBox to rely on the CallCore instead of the
configuration keys content to get a codec description.

This offers more flexibility and the possibility to handle codecs in
different CallManagers in a uniform way wrt the corresponding
CallManager feature.
OpalCallManager: Simplified codecs handling thanks to the new CodecList API.
PrefsWindow: Adapted the new CodecsBox API which requires GmApplication.
TODO: More TODO.
Ekiga: Moved PTRACE intialization to OPAL.

This must be done before the GmApplication arguments parsing. Moreover,
it has nothing to do in the generic code. It belongs to the Opal
component.
Presence: Do not emit a signal in case of error.

We were not handling error cases correctly and were emitting signals
with no presence status, or worse, with error codes in the status
messages.

Opal has changed in a weird way were m_note and such can contain error
messages.
Roster: Fixed transition states and improved code readability.
CodecList: Added a method to check for the presence of a codec.
Ekiga: Prevent Ekiga from running if mandatory codecs are absent.

This will prevent packagers from forgetting codec plugins.
autoconf: Check for Opal AEC support.
schemas: Come with new sane audio & video defaults for codecs.
Presence: Added back support for accidentally removed RFC 4480 states.
TODO: More TODO.
OpalCall: Emit setup after the actual setup.