PaintBrush: Activate tool properly when right-clicking tool button

If we don't go via the action, the tool button becomes checked which
can bypass the exclusivity mechanism. That should probably also be
fixed but it's far outside the scope of where I am right now. :^)

Applications/PaintBrush/ToolboxWidget.cpp

@@ -74,7 +74,7 @@ public:
 
     virtual void context_menu_event(GUI::ContextMenuEvent& event) override
     {
-        set_checked(true);
+        m_action->activate();
         m_tool->on_contextmenu(event);
     }