profile: editor: Also reset the encoding

Add resetting the encoding to default (UTF-8) to the keys reset from the 'reset' button.
encodings: Remove now-unneeded function

Since the encoding ID is now always the same as the charset, remove the ID
and always directly use the charset.
profile: Add key and UI to set the width of ambiguous characters

When using UTF-8 encoding, there is a need to override the default of
ambiguous-width characters being narrow, for compatibility with CJK.

https://bugzilla.gnome.org/show_bug.cgi?id=614618

This requires vte commit 8b7a061714cd6050684b811c3fe5e314b302e417.