WindowServer: Fix visibility of WindowSwitcher constructor

Derivatives of Core::Object should be constructed through
ClassName::construct(), to avoid handling ref-counted objects with
refcount zero. Fixing the visibility means that misuses like this are
more difficult.

This commit is separate from the other Servives changes because it
required additional adaption of the code. Note that the old code did
precisely what these changes try to prevent: Create and handle a
ref-counted object with a refcount of zero.

Userland/Services/WindowServer/WindowManager.h

@@ -432,7 +432,7 @@ private:
 
     u8 m_keyboard_modifiers { 0 };
 
-    WindowSwitcher m_switcher;
+    NonnullRefPtr<WindowSwitcher> m_switcher;
 
     WeakPtr<Button> m_cursor_tracking_button;
     WeakPtr<Button> m_hovered_button;