Migrated protected methods to private implementation

This commit migrates two protected functions in
ContentItemInterface that should not be provided in public
API: emitPropertyChangeSignals and initializationComplete.
Some cleanups

This commit includes some cleanup, like removing useless headers,
reordering includes (alphabetical order, first Qt includes etc.)
reordering members (first methods then attributes) and cleanup
in variable names. It also tries to trim lines in order for
them to be lesser than 100 characters.
Removed friendship between SocialNetworkInterface and ArbitraryRequestHandler

ArbitraryRequestHandler was designed to be an extension of
SocialNetworkInterface, therefore there were a friendship
between those two classes. This commit separates them,
and use signals to trigger signals emissions instead of
calling private members, making the two classes cleaner.
Removed the huge switch for network errors

This commit removed the huge switches that were used
for managing QNetworkReply::NetworkError. It now uses
a shared implementation in util_p.h.

Even if this implementation (that uses metaobjects) is
a bit slower, since it needs more calls, it should not
be a problem.