[tracker] Dicoogle API changes

[Enet4]

Let us use this issue to keep track of all proposed and planned changes to our public APIs when making the transition to a future major version. This looks better than making a milestone because then we have a place to discuss proposals. Also, some major extensions to the API may become available in minor versions if they don't break backwards compatibility.

Planned for v4:

• [ ] #486

In review for v4:

• [ ] Support Dicoogle task registering at plugin level - #146
• [ ] Rethink the index report - #163
• [ ] Correct web service error reporting - #193
• [ ] Capability to extend and access some core classes from Plugins - #128
• [ ] Rearrange plugin service endpoints - #123
• [ ] Storage Middleware Plugins - #255
• [ ] Dynamic Plugin configuration - #103
• [ ] Adjust user authentication for compliance to HTTP - #223
• [ ] Plugin set development improvements - #115
• [ ] Proposal: thumbnail indexer category - #264

Discussion in #11 will also continue here.