Even though Ctrl-U is selected as shortcut for the "Mark as Unread" action (now
even by default, yay), it doesn't work when being pressed.

Steps to reproduce:
1. Make sure Ctrl-U is assigned the "mark as unread" action in the shortcuts
dialog.
2. Select a mail in the message list. (It's now marked as "read".)
3. Press Ctrl-U, and select a nearby other mail so that you can see the
previously selected mail still being black ("read") instead of blue ("unread")
which would have been the expected outcome.

It might be a focus issue, or xmlgui swallowing the shortcut. Might also be some
ugly combination of unfortunate circumstances in Qt/kdelibs communication.
Akregator seems to suffer from the same bug (shortcut for "unread" doesn't work
- and many other shortcuts as well), although I can't tell if Akregator is
suffering from a different issue maybe.

Whatever the cause, this pretty much prevents me from using KMail with keyboard
only, and is a major drawback to my previously established KMail workflow - it's
also a regression compared to the KMail versions that shipped with KDE 3. If you
can fix this, I'll be forever grateful :)