Save user coords with a cache

[following5]

An important feature is missing in OKAPI: Save user-supplied coords / modified coords with a cache.

[following5]

This tricky. The user-supplied coords have different meanings at OCDE and OCPL. But OKAPI treats them just as one thing, the "user-coords"; and lets the client developers decide if they want to treat them as OCDE-style additional waypoint, or OCPL-style modified listing coords. But when submitting coords, OKAPI should be explicit about what they mean!

Also, there is the open question in #455 what these coords should be on the OCPL map - replacement (like in OCPL viewcache) or additional coords (line on OCDE map)?