Clear the visibleImgTags list when setting up the text layout

Since the text layout is being redone then the visibleImgTags will be
repopulated, therefore it should be cleared first so that it doesn't have
duplicated entries.

Change-Id: I415f94326e156ae265bc044544f2bb0d7146ce4c
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>