Make XML tab better

Some useful features would be, in decreasing order of importance:

• clearing all the stanzas;
• freezing the display as it is, with new stanzas being ignored;
• unfreezing, which could either adds the pending stanzas since the freeze if there are few, or just don’t bother with that as it wouldn’t be really useful;
• filter stanzas according to their name, direction, payload namespace (for iq or for all), etc.