[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.
Cleanups, refactors and bugfixes

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().

Some bugs, like ICII not being able to be populated, or UserPicture
not being loaded, are fixed, and there are tests in the Facebook
social test.
Merge pull request #24 from SfietKonstantin/multimodel

[nemo-social] Support multiple models with shared data cache