Added a name sorter

This commit reworks the sorting system, making it to sort
data as QVariantMap instead of items, that are most of the
time null.

It also adds a name sorter, to sort Facebook items that have
a name.
Added cover photo
Behaviour change, no request when no filter

This commit sets a new behaviour for empty filter.
There should not be any request when no filter is provided.
Debug informations
Fixed bug when a ghost item appears in the model

This commit fixes a bug when a ghost item appears
in the model. It was caused by a wrong index
specified in FacebookInterface::beginInsertRows().
The second parameter should have been the index
of the last entry, not the number of entries to
be inserted.