Teach completeion.charset to use CharsetMenu.jsm on Fireofox >= 32

This teaches `:set encoding=…` to work again on Firefox >= 32.

In Firefox 32,
upstream dropped the old Character Encoding menu implementation, [1]
and the charsetTitles.properties, [2]

[1] https://hg.mozilla.org/mozilla-central/rev/9eb4f13a78d8
[2] https://hg.mozilla.org/mozilla-central/rev/15680e55195c

Signed-off-by: Takeshi Banse <takebi@laafc.net>