[mms] IMAP extension enabled information is now stored within the capability object.

Add on error callback to commands that allows for cleanup of internal
state.  For example, unsuccessful ENABLE actions (which actually should
never happen, but never say never) will mark those extension as not
enabled.  Can't use try/catch for this since ENABLE action may be queued.
Actually use the serializable interface
Not needed
property, not parameter
[mms] Querying of valid search charsets is now handled by the Horde_Imap_Client_Data_SearchCharset object.
[mms] Add support for the IMAP UTF-8 extension (RFC 6855).