Add warnings for different visibility settings in the "label-for"
and "labelled-by" elements.

Sine LO doesn't support multiple annotations per accessibility
element, we have to switch them manually, if we change the
visibility of one element of a pair.

Change-Id: I9b3d941248b804b8435591536277677e99b77340
Reviewed-on: https://gerrit.libreoffice.org/61050
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>