Allow valid UTF-8 sequences in the metadata editor

The offending code was added in response to bug #8744. GTK is documented to
expect valid UTF-8 sequences as input, and is free to crash on invalid input
otherwise. The original fix, however, simply replaced non-ASCII characters with
dots which is clearly not accept able for someone named like me. This change
instead skips this replacement when the string is a valid UTF-8 sequence.

The g_utf8_validate must be invoked without the size being passed. If we pass
around the expected size, then this function fails because it hits the first
NUL byte, a terminator, prior to hitting the length limit, and this is
documented to return false.
Update French translation.
Revert "Doc spelling cleanup"

This reverts commit e001c12eaa58f8c1e8bffac96ef8976dbce0be7b.

better read the ~200 new mails before mreging anything: corrections contain mistakes ("automagically") and pick up transatlantic differences (UK/US)
Fix #9534 by cleaning up the filter code a bit

The problem was that "no rejected" was setting the filter to "0+" with
different filter flags. This couldn't get translated back to an entry of
the combobox though, so it got set to "all".
Fix #9604 Add scrollbars to the preferences window
Merge pull request #304 from jktjkt/metadata-allow-utf8

Allow valid UTF-8 sequences in the metadata editor