[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:

• dcm4che migration #486

In review for v4:

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

Discussion in #11 will also continue here.