LibGUI: Add button handler for middle mouse clicks

Userland/Libraries/LibGUI/AbstractButton.cpp

@@ -140,6 +140,9 @@ void AbstractButton::mouseup_event(MouseEvent& event)
             case MouseButton::Primary:
                 click(event.modifiers());
                 break;
+            case MouseButton::Middle:
+                middle_mouse_click(event.modifiers());
+                break;
             default:
                 VERIFY_NOT_REACHED();
             }

Userland/Libraries/LibGUI/AbstractButton.h

@@ -39,6 +39,7 @@ public:
     void set_allowed_mouse_buttons_for_pressing(unsigned allowed_buttons) { m_allowed_mouse_buttons_for_pressing = allowed_buttons; }
 
     virtual void click(unsigned modifiers = 0) = 0;
+    virtual void middle_mouse_click(unsigned) {};
     virtual bool is_uncheckable() const { return true; }
 
     int auto_repeat_interval() const { return m_auto_repeat_interval; }

Userland/Libraries/LibGUI/Button.cpp

@@ -141,6 +141,17 @@ void Button::click(unsigned modifiers)
         m_action->activate(this);
 }
 
+void Button::middle_mouse_click(unsigned int modifiers)
+{
+    if (!is_enabled())
+        return;
+
+    NonnullRefPtr protector = *this;
+
+    if (on_middle_mouse_click)
+        on_middle_mouse_click(modifiers);
+}
+
 void Button::context_menu_event(ContextMenuEvent& context_menu_event)
 {
     if (!is_enabled())

Userland/Libraries/LibGUI/Button.h

@@ -30,12 +30,14 @@ public:
     Gfx::TextAlignment text_alignment() const { return m_text_alignment; }
 
     Function<void(unsigned modifiers)> on_click;
+    Function<void(unsigned modifiers)> on_middle_mouse_click;
     Function<void(ContextMenuEvent&)> on_context_menu_request;
 
     void set_button_style(Gfx::ButtonStyle style) { m_button_style = style; }
     Gfx::ButtonStyle button_style() const { return m_button_style; }
 
     virtual void click(unsigned modifiers = 0) override;
+    virtual void middle_mouse_click(unsigned modifiers = 0) override;
     virtual void context_menu_event(ContextMenuEvent&) override;
 
     Action* action() { return m_action; }