screencast: Add ability to stop streams

When a stream is destroyed by a consumer, mutter won't be able to
recognize that.
For mutter, the stream just paused, but did not disconnect, because the
connection state of a PipeWire stream only represents, whether the
respective PipeWire context is connected to PipeWire.
In addition to that, it may be the case, that the stream consumer just
recreates the stream.
So even if mutter would be able to know, when the stream consumer
destroyed a stream, but not the whole screencast or remote-desktop
session, then mutter would not know, whether the stream will be resumed
eventually or not.

So, add an explicit API call to the screencast interface to stop a
stream.
For virtual streams, this also means, that the respective virtual
monitor is destroyed.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2889
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3307>

data/dbus-interfaces/org.gnome.Mutter.ScreenCast.xml

@@ -199,6 +199,14 @@
     -->
     <method name="Start"/>
 
+    <!--
+        Stop:
+        @short_description: Stop the stream
+
+        Stop the started stream.
+    -->
+    <method name="Stop"/>
+
     <!--
         PipeWireStreamAdded:
         @short_description: Pipewire stream added

src/backends/meta-screen-cast-stream.c

@@ -396,10 +396,35 @@ handle_start (MetaDBusScreenCastStream *skeleton,
   return TRUE;
 }
 
+static gboolean
+handle_stop (MetaDBusScreenCastStream *skeleton,
+             GDBusMethodInvocation    *invocation)
+{
+  MetaScreenCastStream *stream = META_SCREEN_CAST_STREAM (skeleton);
+  MetaScreenCastStreamPrivate *priv =
+    meta_screen_cast_stream_get_instance_private (stream);
+
+  if (!check_permission (stream, invocation))
+    {
+      g_dbus_method_invocation_return_error (invocation, G_DBUS_ERROR,
+                                             G_DBUS_ERROR_ACCESS_DENIED,
+                                             "Permission denied");
+      return G_DBUS_METHOD_INVOCATION_HANDLED;
+    }
+
+  if (priv->src)
+    meta_screen_cast_stream_close (stream);
+
+  meta_dbus_screen_cast_stream_complete_stop (skeleton, invocation);
+
+  return G_DBUS_METHOD_INVOCATION_HANDLED;
+}
+
 static void
 meta_screen_cast_stream_init_iface (MetaDBusScreenCastStreamIface *iface)
 {
   iface->handle_start = handle_start;
+  iface->handle_stop = handle_stop;
 }
 
 static gboolean