Everywhere: Replace Model::update() with Model::invalidate()

Most of the models were just calling did_update anyway, which is
pointless since it can be unified to the base Model class. Instead, code
calling update() will now call invalidate(), which functions identically
and is more obvious in what it does.

Additionally, a default implementation is provided, which removes the
need to add empty implementations of update() for each model subclass.

Co-Authored-By: Ali Mohammad Pur <ali.mpfard@gmail.com>

Userland/Applications/ThemeEditor/main.cpp

@@ -26,7 +26,6 @@ public:
             return Gfx::to_string(m_color_roles[(size_t)index.row()]);
         return {};
     }
-    virtual void update() { did_update(); }
 
     explicit ColorRoleModel(const Vector<Gfx::ColorRole>& color_roles)
         : m_color_roles(color_roles)