[gnome-games/wip/exalm/controller-preferences] ui: Add PreferencesSubpage

[Alexander Mikhaylenko <alexm src gnome org>]

commit b5292094a6a8b60b2fea8ba5b58f67e35aedf770
Author: Alexander Mikhaylenko <exalm7659 gmail com>
Date:   Wed Sep 12 00:51:42 2018 +0500

    ui: Add PreferencesSubpage
    
    This will be used in the next commits to implement subpages in controllers
    preferences.

 src/meson.build                 | 1 +
 src/ui/preferences-subpage.vala | 5 +++++
 2 files changed, 6 insertions(+)
---
diff --git a/src/meson.build b/src/meson.build
index 6c556d47..f1f5302a 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -159,6 +159,7 @@ vala_sources = [
   'ui/preferences-page-plugins.vala',
   'ui/preferences-page-plugins-item.vala',
   'ui/preferences-page-video.vala',
+  'ui/preferences-subpage.vala',
   'ui/preferences-window.vala',
   'ui/quit-dialog.vala',
   'ui/remote-display.vala',
diff --git a/src/ui/preferences-subpage.vala b/src/ui/preferences-subpage.vala
new file mode 100644
index 00000000..64bffcc6
--- /dev/null
+++ b/src/ui/preferences-subpage.vala
@@ -0,0 +1,5 @@
+// This file is part of GNOME Games. License: GPL-3.0+.
+
+private interface Games.PreferencesSubpage: Gtk.Widget {
+       public abstract Gtk.HeaderBar header_bar { get; }
+}