WindowServer+Taskbar: Show applets in taskbar :^)

WindowServer now collects applet windows into an "applet area" which is
really just a window that a WM (window management) client can position
via IPC.

This is rather hackish, and I think we should come up with a better
architecture eventually, but this brings back the missing applets since
the global menu where they used to live is gone.

Userland/Libraries/LibGUI/Event.h

@@ -77,6 +77,7 @@ public:
         WM_WindowStateChanged,
         WM_WindowRectChanged,
         WM_WindowIconBitmapChanged,
+        WM_AppletAreaSizeChanged,
         __End_WM_Events,
     };
 
@@ -108,6 +109,20 @@ private:
     int m_window_id { -1 };
 };
 
+class WMAppletAreaSizeChangedEvent : public WMEvent {
+public:
+    explicit WMAppletAreaSizeChangedEvent(const Gfx::IntSize& size)
+        : WMEvent(Event::Type::WM_AppletAreaSizeChanged, 0, 0)
+        , m_size(size)
+    {
+    }
+
+    const Gfx::IntSize& size() const { return m_size; }
+
+private:
+    Gfx::IntSize m_size;
+};
+
 class WMWindowRemovedEvent : public WMEvent {
 public:
     WMWindowRemovedEvent(int client_id, int window_id)