This simply always underlines the pre-edit text, so it'S easier to
see what text block is currently IM edited.

Change-Id: I45145e4faa872c44eb6583ac6f335dd40fe3e53e
Reviewed-on: https://gerrit.libreoffice.org/65856
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>