Added preliminary support for Gda-ui Vala bindings

* Added --enable-gdaui-vala switch to build Gda-ui Vala bindings
* Deprecated gda_data_entry_contents_is_valid method and ::contents-valid signal
* Added gda_data_entry_validate virtual function to add bindings friendly and
avoid conflicts with ::contents-valid signal and method.
* For now Gda-ui Vala bindings don't build because errors on dependent
Gio-2.0.gir parsing errors. Build depends on Bug 687481.