LibGUI: Implement attributes for windows

Windows can now change whether they have title bar, rounded corners,
alpha channel and whether they are movable. Also windows can also change
their own position

userspace/programs/Terminal/Terminal.cpp

@@ -111,6 +111,11 @@ void Terminal::run()
 	m_fg_color = s_colors_bright[7];
 
 	m_window = MUST(LibGUI::Window::create(600, 400, "Terminal"_sv));
+
+	auto attributes = m_window->get_attributes();
+	attributes.alpha_channel = true;
+	m_window->set_attributes(attributes);
+
 	m_window->fill(m_bg_color);
 	m_window->invalidate();