Add GetMountPoint method for document portal

This is both to get the mount path and to have a nice way
to activate the mount.

data/org.freedesktop.portal.documents.xml

@@ -25,6 +25,9 @@
 
 <node name="/" xmlns:doc="http://www.freedesktop.org/dbus/1.0/doc.dtd">
   <interface name='org.freedesktop.portal.Documents'>
+    <method name="GetMountPoint">
+      <arg type='ay' name='path' direction='out'/>
+    </method>
     <method name="Add">
       <arg type='s' name='uri' direction='in'/>
       <arg type='u' name='doc_id' direction='out'/>

document-portal/xdp-fuse.c

@@ -1724,6 +1724,12 @@ static struct fuse_session *session = NULL;
 static struct fuse_chan *main_ch = NULL;
 static char *mount_path = NULL;
 
+const char *
+xdp_fuse_get_mountpoint (void)
+{
+  return mount_path;
+}
+
 void
 xdp_fuse_exit (void)
 {

document-portal/xdp-fuse.h

@@ -9,6 +9,7 @@ G_BEGIN_DECLS
 gboolean xdp_fuse_init (XdpDocDb *db,
 			GError **error);
 void xdp_fuse_exit (void);
+const char * xdp_fuse_get_mountpoint (void);
 
 G_END_DECLS
 

document-portal/xdp-main.c

@@ -347,6 +347,13 @@ handle_method (GCallback method_callback,
   return TRUE;
 }
 
+static gboolean
+handle_get_mount_point (XdpDbusDocuments *object, GDBusMethodInvocation *invocation)
+{
+  xdp_dbus_documents_complete_get_mount_point (object, invocation, xdp_fuse_get_mountpoint ());
+  return TRUE;
+}
+
 static void
 on_bus_acquired (GDBusConnection *connection,
                  const gchar     *name,
@@ -357,6 +364,7 @@ on_bus_acquired (GDBusConnection *connection,
 
   helper = xdp_dbus_documents_skeleton_new ();
 
+  g_signal_connect_swapped (helper, "handle-get-mount-point", G_CALLBACK (handle_get_mount_point), NULL);
   g_signal_connect_swapped (helper, "handle-add", G_CALLBACK (handle_method), portal_add);
   g_signal_connect_swapped (helper, "handle-add-local", G_CALLBACK (handle_method), portal_add_local);
   g_signal_connect_swapped (helper, "handle-grant-permissions", G_CALLBACK (handle_method), portal_grant_permissions);