Fixe double-fetching in XMPPCollectionModel.

Add a “blockQuery“ boolean member variable to prevent double-fetching
due to delayed fetching. This fixes double fetching the same range of
data from a XMPP entity. Maybe a better solution in QAbstractItemModel.
Delay artist filtering by 100ms.

When an artist is selected in the artist filter view, the
CollectionModel changes instantaneously its query property. It happens
before QML visually changes the current item in the ListView. With the
XMPPCollectionModel, this blocks the GUI until new data is available.