Taskbar: Start working on a taskbar app.

I originally thought I would do this inside WindowServer, but let's try to
make it as a standalone app that communicates with WindowServer instead.
That will allow us to use LibGUI. :^)

LibGUI/GDesktop.cpp

@@ -16,7 +16,11 @@ GDesktop::GDesktop()
 
 void GDesktop::did_receive_screen_rect(Badge<GEventLoop>, const Rect& rect)
 {
+    if (m_rect == rect)
+        return;
     m_rect = rect;
+    if (on_rect_change)
+        on_rect_change(rect);
 }
 
 bool GDesktop::set_wallpaper(const String& path)

LibGUI/GDesktop.h

@@ -2,6 +2,7 @@
 
 #include <AK/AKString.h>
 #include <AK/Badge.h>
+#include <AK/Function.h>
 #include <SharedGraphics/Rect.h>
 
 class GEventLoop;
@@ -17,6 +18,8 @@ public:
     Rect rect() const { return m_rect; }
     void did_receive_screen_rect(Badge<GEventLoop>, const Rect&);
 
+    Function<void(const Rect&)> on_rect_change;
+
 private:
     Rect m_rect;
 };

LibGUI/GEventLoop.cpp

@@ -64,6 +64,7 @@ void GEventLoop::connect_to_server()
     request.greeting.client_pid = getpid();
     auto response = sync_request(request, WSAPI_ServerMessage::Type::Greeting);
     s_server_pid = response.greeting.server_pid;
+    GDesktop::the().did_receive_screen_rect(Badge<GEventLoop>(), response.greeting.screen_rect);
 }
 
 GEventLoop::GEventLoop()

LibGUI/GWindow.cpp

@@ -64,6 +64,7 @@ void GWindow::show()
     request.window.opacity = m_opacity_when_windowless;
     request.window.size_increment = m_size_increment;
     request.window.base_size = m_base_size;
+    request.window.type = (WSAPI_WindowType)m_window_type;
     ASSERT(m_title_when_windowless.length() < (ssize_t)sizeof(request.text));
     strcpy(request.text, m_title_when_windowless.characters());
     request.text_length = m_title_when_windowless.length();
@@ -140,6 +141,11 @@ void GWindow::set_rect(const Rect& a_rect)
     GEventLoop::current().post_message_to_server(request);
 }
 
+void GWindow::set_window_type(GWindowType window_type)
+{
+    m_window_type = window_type;
+}
+
 void GWindow::set_override_cursor(GStandardCursor cursor)
 {
     if (!m_window_id)

LibGUI/GWindow.h

@@ -16,6 +16,12 @@ enum class GStandardCursor {
     ResizeVertical,
 };
 
+enum class GWindowType {
+    Invalid = 0,
+    Normal,
+    Taskbar,
+};
+
 class GWindow : public GObject {
 public:
     GWindow(GObject* parent = nullptr);
@@ -29,6 +35,7 @@ public:
     void set_double_buffering_enabled(bool);
     void set_has_alpha_channel(bool);
     void set_opacity(float);
+    void set_window_type(GWindowType);
 
     int window_id() const { return m_window_id; }
 
@@ -117,6 +124,7 @@ private:
     Vector<Rect> m_pending_paint_event_rects;
     Size m_size_increment;
     Size m_base_size;
+    GWindowType m_window_type { GWindowType::Normal };
     bool m_is_active { false };
     bool m_should_exit_app_on_close { false };
     bool m_has_alpha_channel { false };
@@ -124,4 +132,3 @@ private:
     bool m_modal { false };
     bool m_resizable { true };
 };
-