A new UNDECIDED state is used if multiple cells with differing border
settings are selected.
This prevents additional borders to be set without being asked.
Fixes: #10010
The code in InsetTabular disables the toolbar icons and menu entries related to vertical lines.
The code in GuiTabular disables vertical lines in the Settings dialog.
Note that there was a bug in
GuiSetBorders::set(Left|Right|Top|Bottom)Enabled, where the borders
did not get drawn correctly.
Fixes bug #9816
