Add social network abstraction adapters

Most social networks provide API to allow applications to request or
upload data to their services.  This commit adds adapters which
provide some convenience abstractions to allow various social networks
to be supported, as well as some specific implementations of those
abstractions for some popular social networks.
Cleanup for FilterInterface and SorterInterface

This commit provides small cleanups for FilterInterface
and SorterInterface, splitting files in order to match
header and source files. It also remove the red underline
in QtCreator with the Q_INVOKABLE keyword.
Begin private code cleanup

This commit provides the beginning of private code
cleanup, converting ContentItemInterface and
IdentifiableContentItemInterface to the Qt private
macros.

Currently, code specific to Facebook has been disabled
because they need to be ported as well.
Added properties in IdentifiableContentItemInterface

This commit adds the missing properties in
IdentifiableContentItemInterface.
Merge branch 'social-cleanup' of https://github.com/SfietKonstantin/nemo-qml-plugins into social-cleanup