mconnect's certificate is accessible as Certificate property of
org.mconnect.DeviceManager interface on /org/mconnect/manager object.

Display device certificate when running show-device in verbose mode. To
introspect the certificate run the following:

    mconnectctl show-device --verbose /org/mconnect/device/0 | \
      certtool -i

Previous version of mconnect did not have a 'certificate' key in device section.
When loading such device do not fail instantly but rather log a warning and let
the loading continue further.

Glib TLS support is required to run some of the tests

The code will now generate the private key and certificate during startup.
There is a potential incompatibility with previous releases and the user may
need to unpair the device (in KDE Connect app) and pair again.

Replace OpenSSL helpers with ones based on GnuTLS. GnuTLS is already an indirect
dependency since we require GLib's TLS support. As support for older protocol
versions was removed, we will not need any extra library for handling explicit
packet decryption.

This change will also resolve potential license incompatibility issues.

Introduce support for device channel encryption using TLS. The change is
compatible with current KDE Connect protocol version 7.

Device no longer has a public key, instead the public key was replaced by its
certificate as obtained during TLS handshake.

Device channel connection is now 3 step - first, the initial connection happens,
after which an identity packet is sent to the device, followed by TLS
handshake (including crypto suite negotiation, certificate exchange). Note, KDE
Connect supports TLS 1.0 at the moment. Failure to perform a TLS handshake will
cause device connection to be dropped.

travis: add Ubuntu CI builds

Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>

add device_added, device_removed signals

add 'disallow_device()' function

Affects show-device output.

Hook to connected()/disconnected() Device signals.