Make IrcUserModel::lessThan() consistent with IrcBufferModel::lessThan()

Just pass non-const pointers - what possibly could go wrong? :P All
the internal data structures and all methods take non-const pointers
so this is better to avoid ugly const casts...
Overhaul QDebug stream operators
Added IrcUserModel::aboutToBeAdded/Removed(IrcUser*) signals
Fix IrcUserModel::namesChanged() signal when removing users
Update tst_IrcUserModel::testChanges()