Allow host apps to register themselves

For host apps, we currently require applications to run inside a cgroup
with a name that allows xdg-desktop-portal to derive an application ID
from it. This does not work reliably, and while the situation can
improve, it's unlikely to ever be fool proof, as it in theory requires
every potential piece of software that launches other applications to
know how to launch it in a compatible way.

Cgroups is also a Linux specific API, which is problematic from a *nix
wide portability perspective.

To address this, introduce a host-only portal API that allows
applications to register their D-Bus connections, associating them with
an application ID. This moves back the burden of associating an
application with an application ID back to the application itself,
instead of the application launcher.

Applications calling this overrides any applicion ID derived from the
name fo the cgroup the application runs inside.

data/org.freedesktop.host.portal.Registry.xml

@@ -0,0 +1,57 @@
+<?xml version="1.0"?>
+<!--
+ Copyright (C) 2024 Red Hat, Inc.
+
+ SPDX-License-Identifier: LGPL-2.1-or-later
+
+ This library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ This library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with this library. If not, see <http://www.gnu.org/licenses/>.
+
+ Author: Matthias Clasen <[email protected]>
+-->
+
+<node name="/" xmlns:doc="http://www.freedesktop.org/dbus/1.0/doc.dtd">
+  <!--
+      org.freedesktop.host.portal.Registry:
+      @short_description: Interface to associate D-Bus peers with application
+      ids.
+
+      This simple interface lets unsandboxed applications register their
+      D-Bus connections and associate it with an application ID that will be
+      used in portals.
+
+      This interface will not work with applications xdg-desktop-portal
+      identifies as sandboxed.
+
+      This documentation describes version 1 of this interface.
+  -->
+  <interface name="org.freedesktop.host.portal.Registry">
+    <!--
+        Register:
+        @app_id: Application identifier
+        @options: Vardict with optional further information
+
+        Register a D-Bus peer and associate it with an application ID. The
+        application ID must be able to match the basename of a .desktop file
+        that describes the application.
+
+        The application ID will be used in org.freedesktop.portal APIs to
+        associate a portal action with an application.
+    -->
+    <method name="Register">
+      <arg type="s" name="app_id" direction="in"/>
+      <arg type="a{sv}" name="options" direction="out"/>
+    </method>
+    <property name="version" type="u" access="read"/>
+  </interface>
+</node>