Make PositionedWindow and ShellWindow abstract classes

Author: leolost2605

src/ShellClients/PositionedWindow.vala

@@ -5,7 +5,7 @@
  * Authored by: Leonhard Kargl <[email protected]>
  */
 
-public class Gala.PositionedWindow : Object {
+public abstract class Gala.PositionedWindow : Object {
     public Meta.Window window { get; construct; }
 
     private ulong position_changed_id;
@@ -33,8 +33,5 @@ public class Gala.PositionedWindow : Object {
         SignalHandler.unblock (window, position_changed_id);
     }
 
-    protected virtual void get_window_position (Mtk.Rectangle window_rect, out int x, out int y) {
-        x = 0;
-        y = 0;
-    }
+    protected abstract void get_window_position (Mtk.Rectangle window_rect, out int x, out int y);
 }

src/ShellClients/ShellWindow.vala

@@ -5,7 +5,7 @@
  * Authored by: Leonhard Kargl <[email protected]>
  */
 
-public class Gala.ShellWindow : PositionedWindow, GestureTarget {
+public abstract class Gala.ShellWindow : PositionedWindow, GestureTarget {
     public bool restore_previous_x11_region { private get; set; default = false; }
 
     /**