null_deleter: Added a NULL deleter.

This is commonly used with boost smart pointers.
Action: Added support for templates.

We are supposed to use DataActions with Presentities, Contacts, ... That
is why the old ContactAction/ContactActor classes were renamed and
templatized.
Main window: Replaced deprecated functions.
Main window: Use popover.
Configure: Impose more recent GTK+ release.
ContactAction: Improved code readability thanks to typedef's.
DataAction: Fixed data initialisation.