Review for "improve import/export of line styles"

Hi all,

please have a look at https://gerrit.libreoffice.org/#/c/78372/

An important question is whether you agree that I have changed the 
import and export of our own line styles so that no prstDash line styles 
are used for them. Instead I export them as custDash elements and 
reconstruct them in the import.

I have not added tests yet. Please tell me, what you think would be good 
to test.

You know I always have a hard time with C++. So I welcome hints on how 
to implement something better.

And technical question: What do I need to do locally in Git, so that the 
information, that the no longer needed file "lo_preset_dashes.odt" has 
to be deleted, is included in my commit?

Kind regards
Regina