Add HTTP-POST option, POST message as JSON when enabled

[Muspah]

Basic implementation of HTTP POST'ing messages to an API endpoint.

Adds a dialog via Options -> HTTP POST to configure.
Adds messages to a queue when POST'ing is enabled.
Works through the queue and POSTS's messages to configured endpoint to prevent hanging PDW when POST'ing is having issues.

Handles IP and DNS-based endpoints over both IPv4 and IPv6.
Allows to configure basic auth.

All JSON is converted to UTF-8 before POST'ing.

Small to-do's:

• Format date and time into a valid ISO8601 datetime value.

Wishlist:

• Define custom headers (eg. for API keys)
• Window with some logging and counters

To discuss:

• Adding a setting to only push filtered messages like mail. A paging network will never flood a modern HTTP endpoint. Do we really want to implement all this logic? Is there a large enough use-case for this, even a basic Raspi webserver should be able to handle all messages.

Not implemented by design:

• HTTP GET requests, would add the need to create all kinds of extra logic to be able to define URL params, placeholders like filter cmd, et cetera. HTTP POST is the logical choice with REST in mind.