builder: Add gtk_builder_set_current_object()

builder: Add gtk_builder_set_current_object()

Use it as the default object for expression binds and when connecting
signals. It is intended to work kind of as the "this" object while
parsing. In fact, the term "current object" was stolen from the Java
docs and various C++ tutorials for the this pointer.

Set the current object in gtk_widget_init_template() and
GtkListItemBuilder.

This more-or-less replaces the object passed to
gtk_builder_connect_signals() in GTK3.