IMAP: make GMail happy when invoking ESEARCH

There's currently a bug in GMail's implementation of the ESEARCH command where
they don't correctly handle the SEARCH command with an empty return
specification ("RETURN ()") which the RFC mandates to be equivalent to "RETURN
(ALL)".

I've already reported that to Google and Brandon has promised a fix. Our users
can also add ESEARCH to the blacklisted capabilities in order to make it work,
but this is far from being easily discoverable. Let's waste these three bytes
until GMail rolls out a release with a proper fix -- this is a major
interoperability issue which prevents even basic mailbox resynchronization, so
it's worth the effort IMHO.
added support for saving .eml files
GUI: open the target message after pressing PgUp/PgDown/Home/End as well

Previsouly, only the up and down keys would trigger that. However, at least for
me it was rather counterintuitive having to press Enter after pressing End, so
let's give this a try.
cleanup: better names and minor tweaks
Move related code together
Suggest a proper filename when saving complete messages into an .eml file
Merge branch 'saving-full-messages'

fixes #543