C++: Add Observatory, a first logging UI

This adds Observatory, a first Qt GUI for receiving log messages, filtering them etc. Current features comprise:

• Changing global subscription level (all satellites, all topics)
• Filtering by level, sender, topic, message content (regex)
• Settings are stored and retrieved upon startup

A few things are still open:

• We need to look into this subscription thing, I get erratic results sometimes and don't understand why
• Some UI improvements such as number of connected peers (rename from satellites...), requires upstream changes
• Possibility to change logging per satellite
• Possibility to store to file? Could come later.
• Some performance review - storing all in a vector doesn't seem to be too scalable, maybe limit length?
• Code cleanup, documenting it
• Documentation

Based on !413

PS: this follows the design drawn up at the 2nd EDDA hackathon: