because my new rewriter easily generates overlapping rewriting.

Move the code from flatten and salcall up into the pluginhandler, and
drop the simpler detection logic.

Change-Id: I3da51ac510954a5d4276cee0924cc5dc1fc9a734
Reviewed-on: https://gerrit.libreoffice.org/49493Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>