Use GLib.Once for singletons

lib/Drawing/StyleManager.vala

@@ -36,7 +36,7 @@ public class Gala.Drawing.StyleManager : Object {
 
     private static GLib.Once<StyleManager> instance;
     public static StyleManager get_instance () {
-        return instance.once (() => {return new StyleManager ();});
+        return instance.once (() => new StyleManager ());
     }
 
     public ColorScheme prefers_color_scheme { get; private set; default = LIGHT; }

src/Background/BackgroundCache.vala

@@ -5,26 +5,17 @@
  */
 
 public class Gala.BackgroundCache : Object {
-    private static BackgroundCache? instance = null;
-
+    private static GLib.Once<BackgroundCache> instance;
     public static unowned BackgroundCache get_default () {
-        if (instance == null)
-            instance = new BackgroundCache ();
-
-        return instance;
+        return instance.once (() => new BackgroundCache ());
     }
 
     public signal void file_changed (string filename);
 
     private Gee.HashMap<string,FileMonitor> file_monitors;
     private BackgroundSource background_source;
-
     private Animation animation;
 
-    public BackgroundCache () {
-        Object ();
-    }
-
     construct {
         file_monitors = new Gee.HashMap<string,FileMonitor> ();
     }

src/PluginManager.vala

@@ -5,12 +5,9 @@
  */
 
 public class Gala.PluginManager : Object {
-    private static PluginManager? instance = null;
+    private static GLib.Once<PluginManager> instance;
     public static unowned PluginManager get_default () {
-        if (instance == null)
-            instance = new PluginManager ();
-
-        return instance;
+        return instance.once (() => new PluginManager ());
     }
 
     public delegate PluginInfo RegisterPluginFunction ();

src/Widgets/MultitaskingView/StaticWindowContainer.vala

@@ -12,7 +12,7 @@
 public class Gala.StaticWindowContainer : ActorTarget {
     private static GLib.Once<StaticWindowContainer> instance;
     public static StaticWindowContainer get_instance (Meta.Display display) {
-        return instance.once (() => { return new StaticWindowContainer (display); });
+        return instance.once (() => new StaticWindowContainer (display));
     }
 
     public signal void window_changed (Meta.Window window, bool is_static);

src/WindowDragProvider.vala

@@ -9,7 +9,7 @@
 public class Gala.WindowDragProvider : Object {
     private static GLib.Once<WindowDragProvider> instance;
     public static WindowDragProvider get_instance () {
-        return instance.once (() => { return new WindowDragProvider (); });
+        return instance.once (() => new WindowDragProvider ());
     }
 
     public signal void enter (uint64 window_id);