Fixed the gstreamer plugin compilation with GSettings
Added an experimental Opal::Presentity implementation

This is heavily based on Local::Presentity's code, but with some little
improvements here and there (pushing build_node as a static method makes
a better api, for instance), removed presence fetching/unfetching code
(which will need to be re-added later, perhaps differently).

It hasn't been added to the build system, and was never compiled.
Opal::Presentity: I see no need for that doc parameter!
Opal::Presentity: try to get rid of the need for the whole cluster...
Opal::Presentity: static is only for the declaration in the class
Opal::Bank: first draft of the modification
Opal::Account: first draft
Opal: added the Opal::Presentity code to lib/Makefile.am
Opal::Account: cleaned how the 'type' is handled

In addition, editing the account also sets the type correctly!
Opal::Account: a local variable was shadowing an instance variable
Opal::Account: added the get_groups method
Opal::Bank: finally made it use an XML document, and fixed a few compilation errors
Opal::Account: actually make it link
Main window: remove now unused function (fixes a compilation warning)
Avahi::Heap: fixed a compilation warning by renaming a variable
Loudmouth::Bank: fixed a compilation warning
Renamed the accounts-list schema key to accounts and made it a string
Opal: added the Opal::Bank to the Ekiga::PresenceCore so the accounts become visible
Opal::Account: added the code to add a contact in an account's roster
Opal::Bank: transmit the required signals upward so the gui is updated correctly