Causes this (or one of its descendents) to have the keyboard focus for the Window 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 grab_focus on an already focused widget is allowed, should not have an effect, and return true.
true if focus is now inside this.