Taskbar: Set min/max size for taskbar buttons

This makes them shrink when the taskbar fills with too many buttons.
It doesn't scale to infinity open windows, but it's better than them
escaping off screen after 6 open windows. :^)

Services/Taskbar/TaskbarWindow.cpp

@@ -158,8 +158,9 @@ void TaskbarWindow::on_screen_rect_change(const Gfx::IntRect& rect)
 NonnullRefPtr<GUI::Button> TaskbarWindow::create_button(const WindowIdentifier& identifier)
 {
  auto& button = main_widget()->add<TaskbarButton>(identifier);
- button.set_size_policy(GUI::SizePolicy::Fixed, GUI::SizePolicy::Fixed);
- button.set_preferred_size(140, 22);
+ button.set_size_policy(GUI::SizePolicy::Fill, GUI::SizePolicy::Fill);
+ button.set_min_size({ 20, 22 });
+ button.set_max_size({ 140, 22 });
  button.set_text_alignment(Gfx::TextAlignment::CenterLeft);
  button.set_icon(*m_default_icon);
  return button;