make context menu better manageable by devs

[r10s]

currently, there is a little abstraction layer for the messages' context menu, that does not help much or avoid double code, but makes "nicer" things harder -eg. having a separator, submenus, or preferredElementSize

let's make the code there much simpler, and split only between "current iOS, having all nice menu options" (that would be ios15) and a fallback that works also on ios12 (using the old, text-edit-like menus without blurring)

for reactions, it is fine to offer just a simple "react" for ios14 and below that opens the "any emoji selector" (currently, we do not have reactions at all for ios12)

@zeitschlag please correct me if i understood sth wrong :)