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.
Bugfixes

Bug fix when having an empty model that causes crashes.
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.
Adaptation for Friends-QML