Current background color function only sets table cell background when
multiple table cells are selected. Paragraph background is otherwise
set. This gives expected results for use in sidebar Paragraph panel when
multiple table cells are not selected but unexpected results when
multiple table cells are selected. For background setting from the table
tool bar unexpected paragraph background color is set when multiple
cells are not selected.

This fix creates a table cell background color uno command specific for
setting table cell background color.

Change-Id: Ic56723b635252415d932c5499f996c3e46eee13b
Reviewed-on: https://gerrit.libreoffice.org/65591
Tested-by: Jenkins
Reviewed-by: Heiko Tietze <tietze.heiko@gmail.com>