Fix asserts due to the use of invalid fonts in HTML viewer.

Don't change the font style/weight if we don't have a valid font, this results
in asserts from wxFont::GetWeight().
Fix fatal crash due to c-client reentrancy when reading messages.

If the mail collection timer expired at exactly the same time when we were
deleting the progress dialog used for showing the progress of receiving big
messages data, we could crash because of c-client reentrancy. Fix this by
preventing background processing during the dialog deletion.
Update versions in the user-visible files to 0.68.

No real changes, just synchronize the versions everywhere.
Updated the year to 2013 in user-visible places.
Minor roadmap updates after 0.68.

Don't mention some items that are already done, postpone most of the other
ones.
Replace the mention of cvs with git in release instructions.
Pre-fill Mahogany users mailing list address in autocollect address book.

Also install it to the user Mahogany directory under Windows.
Build Windows releases with MSVS 2010.

This makes it much simpler to distribute the binaries as the CRT DLLs can be
just copied into the application directory again since VC10 (as it was the
case with VC7 but not VC8 and VC9).
Updated year in splash image too.
Fix possible crash when parsing malformed RFC 2047 header encodings.

We were reading beyond the end of the string if an encoded quoted word wasn't
correctly followed by "?=".