WindowServer: Vertically maximize window

Button now can handle middle and right clicks.
Added 2 new handlers in button class: on_right_click for Right mouse
button and on_middle_click for middle mouse button.

Added functionality to vertically maximize window with middle mouse
click on the maximize window button.

Also added a way to vertically maximize window by resizing window
height-wise lower than the maximum window height.

Userland/Services/WindowServer/WindowFrame.cpp

@@ -83,6 +83,9 @@ WindowFrame::WindowFrame(Window& window)
         auto button = make<Button>(*this, [this](auto&) {
             WindowManager::the().maximize_windows(m_window, !m_window.is_maximized());
         });
+        button->on_middle_click = [&](auto&) {
+            m_window.set_vertically_maximized();
+        };
         m_maximize_button = button.ptr();
         m_buttons.append(move(button));
     }