[Matrix] few cleanups on addon and version increase to 19.8.1

This add a github action to make for build tests a Debian package creation
On "disclaimer" in addon.xml.in is nearly by all addons with an c&p a wrong ".." on end, this removes it
By strings.po about "en_gb" was some set on "msgstr" where sould be empty there.
- As they has looked mostly like a help text becomes it separated to new strings and used as help on related setting.
As this addon mostly used by germans, has added a german translated strings.po, as there currently no outside place where does it (e.g. weblate)
Taken about rapidjson some patches from debian to prevent this big warning show during build.
- Because the hell they can't get a new release at https://github.com/Tencent/rapidjson :roll_eyes::rage:
Also is the settings.xml changed to new format

WARNING: Due to missing account, only tested indirect about changed settings (without connection usage)!

rbuehlma / pvr.teleboy