MessageWindow: Add back tab scrolling

This feature was intentionally removed in GTK, see
https://bugzilla.gnome.org/show_bug.cgi?id=630226
3 jobs for tab-scroll in 2 minutes and 29 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #13828
run-appdata

00:00:07

passed #13827
run-tests

00:02:11

 
  Build
passed #13829
run-build

00:00:11