ActivityListModel: fix possible crash in slotActivitiesReceived

ActivityListModel: fix possible crash in slotActivitiesReceived

Backtrace from the crash reporter:

Crash: EXCEPTION_ACCESS_VIOLATION_READ at 0x401
  File "moc_activitylistmo_M5OEXJ7XGJYTWT.cpp", line 92, in OCC::ActivityListModel::qt_static_metacall
  File "qobject.cpp", line 3730, in QMetaObject::activate
  File "moc_networkjobs_7AMNCW4BBANVRK.cpp", line 1342, in OCC::JsonApiJob::jsonReceived
  File "networkjobs.cpp", line 714, in OCC::JsonApiJob::finished
  File "abstractnetworkjob.cpp", line 207, in OCC::AbstractNetworkJob::slotFinished
  File "moc_abstractnetwor_PFI2TXGQHRE33H.cpp", line 98, in OCC::AbstractNetworkJob::qt_static_metacall
  File "qobject.cpp", line 3730, in QMetaObject::activate
  File "moc_qnetworkreply.cpp", line 367, in QNetworkReply::finished
  File "qnetworkreplyhttpimpl.cpp", line 2100, in QNetworkReplyHttpImplPrivate::finished
  File "qnetworkreplyhttpimpl.cpp", line 279, in QNetworkReplyHttpImpl::abort

My theory is that the AccountState stored in a property of the job was destroyed.
before the job timed out.
Therefore, the qobject_cast within the qvariant_cast would call the metaObject()
virtual function on a dangling pointer.

Fix it by storing a QPointer instead which will track the deletion.