systray: tweak the font size, color and transparency to produce a reasonable result

This is black magic, but it looks like these values work better than the
previous ones. It was evil to try to use the user's color scheme when working
with an icon which has hardcoded colors.
GUI: use the systemwide monospace font in the signature editor
Settings: don't default to launching QProcess by default, assume SSL

This is mainly just a usability change -- most users are likely to connect via
SSL (or TCP), not via IMAP-over-SSH or something fancy like that. Let's make the
configuration process tad easier.
Settings: don't save obviously incomplete settings for the IMAP account
Make it clear to the user than one cannot do much without a properly configured IMAP account

Thanks to Steffen Schaumburg <technik@pannonia-isd.de> (catpig on IRC) for his
bug report about the old behavior not being terribly user-friendly.
GUI: disable sending of e-mails without any subject

Nope, I don't want to add a button which would make it possible to send a mail
with an empty subject.
GUI: unbreak saving of individual message parts and add proper menu for saving complete messages

Also instantiate the download managers on demand to cut down the excessive
memory use.

This has apparently been broken since I merged the patch which added the
FullMessageCombiner. Thanks to Caspar for his bugreport.

fixes #643