The behavior that cursor jumps out of current table box on Del/Backspace
introduced in commit 80a4b3b5 is
not intuitive, and differs from what other word processors do.

Unit test included.

Change-Id: Icb53b6733f0d7394abe011fa067089e6693cf648
Reviewed-on: https://gerrit.libreoffice.org/49257Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>