This is a telepathy authentication handler using libaccounts and libsignon,
kindly borrowed from Empathy's ubuntu-online-account support. Along with a
mission-control plugin (not included) providing a bridge between libaccounts
and telepathy, this allows proper use of libaccounts and libsignon to control
telepathy accounts.

The code is deliberately kept mostly similar to the upstream empathy code for
now, since there's limited effort towards maintaining it separately.

The auth handler will be invoked by telepathy when needed to authenticate a
connection. That account is looked up with libaccounts, and then its credentials
are found from libsignon, and used along with various SASL mechanisms to sign
into the account.

In Empathy, this component also displays a dialog when credentials fail and need
to be re-entered, and supports updating those credentials in libsignon. That
functionality does not exist here, and all other GTK usage was removed.

In Empathy, this component is also responsible for verifying TLS certificates,
and displaying a dialog to the user to interactively bypass failing certificates.
Without the dialog, that functionality seems redundant to what telepathy CMs provide,
so it is also clumsily disabled but left largely intact in case it's needed later.

qmake is used for build as a convenience. There is no other Qt dependency at present.

Code is forked off of Empathy git at EMPATHY_3_7_2-10-ga89694c