Add mutators as `PRODID` parameter

[rfc2822]

Previously, we had the mutators (apps that modified the local event) in the PRODID value like this:

PRODID:DAVx5/... ical4j/... (app1.package, app2.package)

This is very useful because it's not DAVx5 alone that generates an event, but the data comes from the app and potential problems can be related to a specific calendar app. So it's very useful to see which app provided the information for the generated iCalendar.

However, it's sometimes a problem that the PRODID has a variable length: bitfireAT/synctools#141. So in DAVx5 4.5.6, we drop the app package names as a quick workaround.

It's also a bit confusing that the package names of the editor are in parentheses after DAVx5 because it looks like if that would be the DAVx5 package name.

However a separate iCalendar property (X-MUTATORS:app1,app2) can't be used because other clients would retain the property, which would make it useless.

So a possible solution would be to

• include the mutators as a x-param of the PRODID, and to
• add an optional x-param if then PRODID line length is exactly n*75 to avoid the "empty line through folding" problem until Properties are folded when line is exactly max line length ical4j/ical4j#832 is fixed

like that:

PRODID;x-mutators=app1,app2:DAVx5/... (ical4j/...)

and if that would be exactly 75 chars, then instead:

PRODID;x-mutators=app1,app2;x-avoid-empty-line=1:DAVx
 5/... (ical4j/...)

However this is a big change and we should check whether all major servers accept such properties.

TODO: Compatibility table