fdo#73617 sc: Don't update the gui during live preview

Apart from (apparently) being not necessary, it creates
bugs like this one. If the selection consist of multiple
fonts, it leads to clearing the font name box, which in
turn leads to scrolling the list up.

Change-Id: I8e702b9b4d4a1f7ad1ded5de1a5e162f039f6d4d