IPC: set ws reply "num" member to -1 when named

Long description:

When a named workspace (i.e., a workspace that has a name that does not
begin with text that can be parsed as an integer greater than or equal
to zero) is represented by the ipc as a workspace json object such as
can be queried with `i3-msg -t get_workspaces`, set the num property to
-1 instead of json null.

This is for convenience of ipc consumers using type-constrained
languages such as C which have difficulty cleanly expressing nullable
integers.

fixes #1368

To apply this patch, use:
curl http://cr.i3wm.org/patch/644/raw.patch | git am

@@ -156,7 +156,7 @@ following properties:

 num (integer)::

 	The logical number of the workspace. Corresponds to the command

-	to switch to this workspace.

+	to switch to this workspace. For named workspaces, this will be -1.

 name (string)::

 	The name of this workspace (by default num+1), as changed by the

 	user. Encoded in UTF-8.

@@ -617,10 +617,7 @@ IPC_HANDLER(get_workspaces) {

             y(map_open);

             ystr("num");

-            if (ws->num == -1)

-                y(null);

-            else

-                y(integer, ws->num);

+            y(integer, ws->num);

             ystr("name");

             ystr(ws->name);

@@ -24,7 +24,7 @@ sub check_order {

     my ($msg) = @_;

     my @ws = @{$i3->get_workspaces->recv};

-    my @nums = map { $_->{num} } grep { defined($_->{num}) } @ws;

+    my @nums = map { $_->{num} } grep { $_->{num} != -1 } @ws;

     my @sorted = sort @nums;

     is_deeply(\@nums, \@sorted, $msg);