data: Move D-Bus interface descriptions into a subdir

We'll soon move the inlined XML of all interfaces we export or consume into
separate files. That's a significant increase from the interfaces for which
we install documentation, so it makes sense to use a dedicated subdir.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/537

data/dbus-interfaces/meson.build

@@ -0,0 +1,8 @@
+dbus_interfaces = [
+  'org.gnome.Shell.PadOsd.xml',
+  'org.gnome.Shell.Screencast.xml',
+  'org.gnome.Shell.Screenshot.xml',
+  'org.gnome.ShellSearchProvider.xml',
+  'org.gnome.ShellSearchProvider2.xml'
+]
+install_data(dbus_interfaces, install_dir: ifacedir)

data/meson.build

@@ -41,16 +41,7 @@ foreach service_file : service_files
 endforeach
 
 
-dbus_interfaces = [
+subdir('dbus-interfaces')
-  'org.gnome.Shell.PadOsd.xml',
-  'org.gnome.Shell.Screencast.xml',
-  'org.gnome.Shell.Screenshot.xml',
-  'org.gnome.ShellSearchProvider.xml',
-  'org.gnome.ShellSearchProvider2.xml'
-]
-install_data(dbus_interfaces, install_dir: ifacedir)
-
-
 subdir('theme')
 
 theme_resources = gnome.compile_resources(