With shift modifier, all rows/cols should get deselected upto the
current cursor position from the last cursor position *if* the
last cursor position *with* KEY_MOD1 resulted in a deselection of
row or column.

Added a unit test to support this.

Change-Id: I7b338ca52505d59480209802ee65a6d64b0ac67d
Reviewed-on: https://gerrit.libreoffice.org/23364Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>