Call Ring ID accepted as command line argument

Allows gnome-ring to accept as the first command-line argument and
automatically call a target RingID in the form of 'ring' URI scheme:
"ring:[0-9a-z]{40}".

It is possible to use filename and URI parsing capabilities of
GTK applications to extract RingID from command-line argument
if it is provided.

This patch implements the 'open' hook of the GTK-application,
as recommended by GTK documentation, to correctly communicate data
between multiple running instance of the application. By doing this,
the following behavior is achieved:

1. If gnome-ring is not yet running and it is invoked with
a command-line argument, a new application instance will be initialized
and will start a new call at once.

2. If gnome-ring is already running, the 'open' event with the RingID
will be dispatched to the primary application instance and make it start
a new call.

= Testing the patch

Apply the patch, recompile gnome-ring and execute it
with the command-line argument in the Ring URI-scheme format:

gnome-ring 'ring:0000000000000000000000000000000000000000'

= Expected behaviour

The main application window will appear and automatically start
a call to the destination RingID.

[GR: fix ci msg line lenght (limit to 72)]
[GR: fix naked raw ptr and build warnings]

Change-Id: I109f118fb4765e764dc399486091e456ef19117e
Tuleap: #1539