Fix the build with the lastest GTK+ from git master.

* gtk/src/gtk_signals.defs: Update this with
tools/gen_scripts/gtk_generate_extra_defs.sh .
* gtk/src/entry.hg:
* gtk/src/textview.hg: Add _CONVERSION lines to cope with the slightly-changed
GTK+ API caused by this commit:
https://git.gnome.org/browse/gtk+/commit/?id=88623b98a7ca0afca08ac21bc3aa662c6ed3ab7f

Bug #695305 (Cosimo Cecchi, mussadek)