WindowServer: Update window switcher when moving window to front

If the window switcher is up when a window is made frontmost (usually
by clicking on it), the window will now become the selected window in
the switcher.

This still has a slight feeling of "hmm" but is definitely better than
what we had before.

Servers/WindowServer/WindowSwitcher.h

@@ -68,6 +68,8 @@ public:
     int padding() const { return 8; }
     int item_padding() const { return 8; }
 
+    void select_window(Window&);
+
     Window* selected_window();
 
     Window* switcher_window() { return m_switcher_window.ptr(); }