Telepathy accounts support

This adds support for reading account data from Telepathy, in its KDE flavor (i.e. when creds are in KWallet).

It is not an importer, it loads accounts from Telepathy on each startup and keeps them read-only, with the option of disabling them.

Quirks:
* Changes to KTp account settings are only applied to Tomahawk on its next run... still not sure how to handle this differently or if it's even worth it.
* One cannot permanently remove a KTp account from Tomahawk, it must be removed globally from the KTp KCM. We could support blacklisting an existing KTp account from Tomahawk, but again I'm not sure if it's worth it and how to present the reverse operation (un-blacklisting).

New optional runtime dependencies:
* TelepathyQt
* KDElibs for the KTp KCM