[feature] Multimodel changes

This commit introduces the multimodel API, that separates
the SocialNetworkInterface into two classes. One, still
called SocialNetworkInterface, is used to makage the cache
and interfaces with given social networks. The other class,
SocialNetworkModelInterface, is used to carry data to be
displayed. It uses the cache in SocialNetworkInterface to
get these data.
Finishing touches and bugfixes

This commit provides the finishing touches, like managing model
and filter deletion, sorting, and bugfixes to the Qt5 version of
the test.
Small refactoring

This commit includes some small refactoring, like
reordering methods, warning fixes and
methods / variables renaming.
Small redesign of the relation between SNI and SNMI

This commit includes a small redesign of the relation between
SNI and SNMI. Node classes do not contain the model they are
representing, nor the QNetworkRequest that they are using
to get data, therefore respecting their role of "pure data" class.

Models and replies are now stored by SNIp. Updates are still
done by SNIp as well.

This commit also includes the feature to reload a node, that can
be done via SNMI::repopulate().
Facebook cleanup and UserPicture implementation

This commit includes a small relayout and cleanup of Facebook
implementation, as well as implementation of the UserPicture
filter, that was ignored before.