widget: Use G_PARAM_EXPILCIT_NOTIFY
Revert "widget: Narrow the set of word chars"

This reverts commit 1b6cf784afab99aea861128f5e7bb5619288d6c1.
widget: Simplify word-chars

Until we implement full UAX29 word boundary detection, let's simplify the
exception cases of ascii punctuation we don't want to include.

https://bugzilla.gnome.org/show_bug.cgi?id=730632
widget: Improve set_encoding API

Since opening the converter can fail, make this return a gboolean and a GError.
And change the meaning of NULL as @codeset from 'current locale' to 'UTF-8'.