Threading: prevent dangling persistent indexes when removing multiple messages

Commit faea6a3132fe78fd25e84e7adeeb3edddb89b46b introduced a regression -- when
multiple messages were selected, marked as deleted and subsequently expunged,
the selection reappeared and included new messages which were now at the same
(or similar?) place. This patch fixes one corner case which might have led to
this behavior.

REVIEW: 110705
SVN_SILENT made messages (.desktop file)
Always build the DE detection even on non-X11 platforms

The main motivation behind this is that there's no Q_WS_* in Qt5. Also, checking
a couple env vars on, say, windows shouldn't do any harm.
GUI: Try to use a reasonable font size even on systems with no default config

REVIEW: 110732
Merge branch 'monospace-font-selection'
build: accept the update of auto-generated files from QtCreator
Fix a segfault when going back from the "one at a time" layout and other related bugs

Thanks to Brian Jackson <iggy@theiggy.com> (iggy on IRC) for a bugreport.

REVIEW: 110736
GUI: fix the size of the inner widgets when exiting the "one at a time" mode
GUI: use RAII for the one-at-time layout instead of manual slot setup and other stuff
GUI: make the history navigation within "one at time" layout a bit more robust
Merge branch 'threading-persistent-indexes'
abook: don't offer unrecognized addresses

The perfect fix would be, of course, to make the MailAddress::fromPrettyString
support even "foo@example.org <foo@example.org>", but that look like a lot of
work (and besides that, I don't want to see these regexps in the next month or
so)

BUG: 320499
REVIEW: 110740
Composer: support address completion even on chunks starting with ".", "_" and "-"

REVIEW: 110738
www: link to KDE bugzilla for bug reports

There's now also a server-side redirect which prevents creating new issues
within Redmine. I don't want to "close" it because the history is clearly
valuable.
www: clarify the list of build requirements
Merge branch 'inner-address-completion'
GUI: use reasonable section size hints and remember the user's actual values

The problems which make this non-straightforward:
- the interaction with MsgListView::slotFixSize was funny
- once the size of a section is set to 0, it cannot be resized by user

Reported by Len Nordstrom <mournblade@gmx.us> (len on IRC).

REVIEW: 110704
Merge branch 'fix-qtreeview-section-size-persistency'
Merge branch 'fix-segfault-one-at-time'
EnvelopeView: move signal forwarding setup to the constructor

Looks like I preferred a manual signal propagation at the time I wrote this --
but that's also something which puts excessive requirements on callers. I
believe that this is a better way.