New webxdc api

[nicodh]

resolves (partly) to #4366

Main changes:

• openWebxdc in frontend expects a message as first argument instead a messageId
• selfAddr in webxdcInfo holds not the users address anymore
• WebxdcInfo message has a new optional property webxdc_href
• click on WebxdcInfo message directly opens the map and sets a event related location.url in iFrame (optional)

[nicodh]

Needs next core release

[link2xt]

Core 1.151.3 is published

[WofWca]

I left a bunch of questions. Hate to be that guy, but this MR affects security-critical code.

Overall I think we need more comments. Clearly document function contracts and parameters, are they sanitized or not. If not, how they need to be sanitized. The assumptions, etc.

[nicodh]

Hate to be that guy,

No worries. I appreciate critical feedback! :-)

[WofWca]

I left a comment about the "internetAccess" warning. Please check it out and resolve as you wish.
Otherwise it's good, though ideally I'd like more comments, and get rid of exposeInMainWorld for setLocationUrl. But this can be done later.
IMO This is ready to be merged. (but FYI a CHANGELOG entry is missing)