Moved shift+tab check to the keyPressed function to provide continuous insertion
and compiled tab-pattern only once

Change-Id: Iffe847fea07295075dc1a280f0697e8c798e8d6f
Reviewed-on: https://gerrit.libreoffice.org/72580Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>
Tested-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>