unicode: Simplify width table generation

Move width table generation into the gen-unicode-tables.pl script. This makes
updating the tables automatic without the previously required manual editing
required to insert the tables in the right place of the source code.
unicode: Move gscripttable.h generation into main script

So we just have to run one script when updating the unicode data, not two.
unicode: Switch compose_second_single to gunichar

This will be required for the update to unicode 7.0.0.
unicode: Update to unicode 7.0.0

See bug https://bugzilla.gnome.org/show_bug.cgi?id=731929.