Veusz: fixing #903450

[Jeremy Sanders]

Hi -

I've updated Veusz to try to fix 
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=903450

I've added a breaks+replaces for veusz<<3.0. I've also added one in 
python3-veusz.helpers for veusz-helpers.

Is this the right thing to do? I'm concerned it might end up replacing 
veusz with python3-veusz, and not upgrading the main veusz package.

I've also managed to drop the xvfb build dep. With some changes in the 
build, it also requires qt5-make, so I've added that.

Thanks very much

Jeremy