XEP-0428: Fallback Indication v0.2.0

[mar-v-in]

This incorporates most of the ideas of the "Compatibility Fallback" ProtoXEP as discussed in the council chat room back then.

• Add 'for' attribute such that entities can discover what the fallback is for.
• Allow to specify that only one of <subject/> or <body/> is meant as a fallback.
• Allow to specify the part of respective text that is meant as fallback where applicable.
• Don't use encryption example, which should use XEP-0380 instead.

[mar-v-in]

@dwd

[cal0pteryx]

@mar-v-in congratulations on taking co-authorship on XEP-0428! I'd love to see this move forward, especially in conjunction with XEP-0461: Message Replies 👍

[singpolyma]

Is end intended to be inclusive or exclusive? By the example provided it seems exclusive, but the normative text should indicate one way or the other IMHO.

[mar-v-in]

@singpolyma I'll add that to XEP-0426 instead (as it should be a general convention and not specific to this XEP). (#1260)

[mar-v-in]

Note to editors: This can be merged.