Allow specifying the XML encoding property

Lets see if we can set the encoding to be enabled by default for new installs, and otherwise keep things as they are for everyone until 8.x-2.x.

I've added the start of this. Still need a post update hook that'll update all existing views to set their encoding to the empty option, which will then preserve what they're doing already.

Sorry about the noise there, was on a machine without access to running PHP unit!

Anyway, I think this is ready to be committed now.

steven jones → credited xperd → .

Going to credit users who contributed to ✨ Changing default "" and "" tags in XML export Needs review relating to encoding.