Use a single timer to make sure relative email dates get updated

Move the timer from ConversationListStore to MainWindow, ensure that
both the list and the viewer get their dates updated, but only when
the main window is visible.
5 jobs for <span class="ref-name">wip/fix-date-handling</span> in 29 minutes and 9 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #251902
fedora

00:03:45

passed #251903
ubuntu

00:07:39

 
  Test
passed #251904
flatpak

00:19:21

 
  Deploy
passed #251905
review

00:00:56

passed #251906
allowed to fail manual
stop_review

00:01:11