message dialog: Stop hardcoding title styles

Instead, use a new title-2 style class to let
themes influence title formatting. Note that
the theme style will be overridden if the
application uses markup for presentation,
such as <b> or <i>.
4 jobs for message-dialog-title-for-3 in 11 minutes and 21 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #314302
fedora-autotools

00:11:20

passed #314303
fedora-meson

00:08:09

manual #314305
win32 allowed to fail manual
msys2-mingw32-autotools
passed #314304
win32
msys2-mingw32-meson

00:05:28