IMAPX: Explicitly pass GCancellable and GError to callbacks.

IMAPX: Explicitly pass GCancellable and GError to callbacks.

Further error handling improvements.  This involves some public API
changes, but the impact is limited to evolution-kolab.

 * CamelIMAPXCommandFunc now takes a GCancellable argument in addition
   to its GError argument.

 * The public GCancellable member of CamelIMAPXJob is now private, and
   the public GError member is removed.

 * The start() method of CamelIMAPXJob now takes a GCancellable and
   GError argument.  The GCancellable pass through this method call
   is always the CamelIMAPXJob's own private GCancellable.

 * A new camel_imapx_job_cancel() function which simply cancels the
   CamelIMAPXJob's internal GCancellable.

Additional changes internal to CamelIMAPXServer are:

 * Adapt operations to the new callback function signatures.

 * imapx_command_queue() now takes a GCancellable and GError argument
   and returns a 'success' boolean.  Other core job and command routing
   functions also now have similar function signatures.

 * Add error handling where needed, so that no failable function call
   goes unchecked.