We need to keep fuzzy flag and previous source string over unit updates
if translation has not changed (user can still manually unflag unit as
fuzzy). Fixes #118

On source string, we want to set fuzzy flag as well (issue #118).

On source string change (and if format does not expose this on it's
own), we store current string as previous string, so that translators
can see what has been changed (see issue #118).

With monolingual files, source string can change without changing
checksum, we need to properly detect this and update unit if needed.

See also issue #118

This reverts commit ce1fdc85.

Accepting suggestion now has different entry in changes database.

Currently translated at 57.5% (458 of 797)

Currently translated at 100.0% (797 of 797)

It was not that useful as I hoped for.

While this is faster with smaller amounts of data, it slows down with
huge tables.

Currently translated at 98.6% (786 of 797)

Currently translated at 100.0% (797 of 797)