Diverse Refactoring, error fixing and new features

[Lord-Grey]

Summary

Added

• Support to freely select source and target instances to be used by forwarder
• Systray: Support multiple instances
• UI: Validate that key ports do not overlap across editors and pages

JSON-API

• Support to query for a dedicated set of configuration items for a set of instances
• Support to save a dedicated set of configuration items for a set of instances
• Limit update emission frequency: Images (25Hz), raw LED-Colors (40Hz) & LED-Device data (200Hz)

Changed

• Removed hard dependency on the first instance. All instances can now be freely created/removed, started or stopped
• Refactored: Forwarder
• Refactored: Flatbuffer connection- and client handling
• Refactored: Decouple Effect definitions from Instance
• Refactored: Decouple WebServer and SSDPHandler
• Refactored: Corrected thread affinity across various classes
• Refactored: Improved code resilience and error handling
• Standalone grabber do not capture screens when no connection to remote host
• Fixed mDNS Browser deadlock, plus run in own thread now
• Fixed that LED Buffer and Layout might get out of sync.
• Fixed Provide custom forwarding targets is not possible (Manual set of flatbuffer forwarding target is unavailable #1713)

JSON-API

• Configuration requests do not any longer require a running instance
• Fixed: Create a proper API response, when Effects are not part of a build

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of web configuration, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing setups:

The PR fulfils these requirements:

• When resolving a specific issue, it's referenced in the PR's body (e.g. Fixes: #xxx[,#xxx], where "xxx" is the issue number)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated (docs/docs/en)
• Related tests have been updated

PLEASE DON'T FORGET TO ADD YOUR CHANGES TO CHANGELOG.MD

• Yes, CHANGELOG.md is also updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:
fixes #1713