Completion: simplify the hiding of the calltip

It is now done by GtkSourceView.
Autotools: use upstream macro AM_PROG_VALAC

The bug has been fixed since Automake 1.12.5.
Autotools: don't use INCLUDES (deprecated)
Completion: disable accelerators (Alt + 1, Alt + 2, etc)

They don't work, anyway, since Alt + N is already used in the Documents
menu.

And with the compact completion window, when "No matching proposal" is
displayed, the "1" on the right is a bit strange.