Causes this to have the keyboard focus for the `GtkWindow` it's inside.
If this is not focusable, or its
GtkWidget::grab_focuss implementation cannot transfer the focus
to a descendant of this that is focusable, it will not take focus and false
will be returned.
Calling [method@Gtk.Widget.grab_focus] on an already focused widget is allowed, should not have an effect, and return true.
true if focus is now inside this.