fdo#79602: sw: add new compatibiltiy flag PropLineSpacingShrinksFirstLine

This is enabled by default, to get the new formatting where the first
line of a paragraph is shrunk if a proportional line spacing < 100% is
applied; existing OOo documents get the previous (before LO 3.3)
formatting.  Since the formatting in LO releases is broken anyway, it
does not matter much which way documents written by old LO get
formatted.

(cherry picked from commit 9605763e3dc8c85137787c77c31e8639553a35ed)

Conflicts:
	sw/source/filter/ww8/ww8par.cxx

Change-Id: I0952f568a933c137bd03070759989cac3517d8b9
Reviewed-on: https://gerrit.libreoffice.org/12157
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>