Vfuncs

[vixalien]

This PR allows the setting of vfunc functions. These are "virtual functions" that will usually be called as the default callback for example, a signal or a method. By allowing setting, we effectively allow subclassing them.

This pattern can be used, for example, to allow adding a default handler for a GtkApplication using the following pattern:

class App extends Gtk.Application {
  // code...
}

const app = new App();

app.vfunc_activate = () => {
  console.log("activated...");
  this.show_main_window();
}

You can see that this is similar to adding a connect handler. This is like setting a function. For example, when we set the activate vfunc, it's as if we're setting the function that executes when the function is executed.

There is also a pattern typical in GJS that allows overriding vfuncs directly, but I still need to implement that.

class App extends Gtk.Application {
  // code...
  vfunc_activate() {
    // create and show the main window
    this.show_main_window();
  }
};

This PR will also help implement GtkBuilderScope's vfunc_create_closure, which is used to make sure callbacks from GtkBuilder template files (XML) are correctly attached to the JS-side callbacks. I will include the code in a future PR (probably #20)

I also need to log a warning or error if a vfunc cannot be set.

[vixalien]

There is also a pattern typical in GJS that allows overriding vfuncs directly, but I still need to implement that.

I just realised that this cannot work currently, because if you try to subclass a type right now (for example, class App extends Gtk.Application) it won't work because the custom class has not been registered with GObject. I will include that in a separate PR, possibly #20 or another PR after that.

I also need to log a warning or error if a vfunc cannot be set.

Same as the one above, if you are trying to override your custom class' vfunc, the checking code will be included as part of the aforementioned future PR. However, setting a random vfunc_ like app.vfunc_blablabla will currently just work fine, and it will set a custom javascript property named like that on the object.

[vixalien]

Please take a look

[ahgilak]

LGTM. Thanks.