Implement the DIP17 images spec

[philpax]

https://github.com/goatcorp/DIPs/pull/45/files

Add a new Images section to the DalamudPlugin project extension that describes where images can be found. This will then be loaded by Plogon and used to populate PluginDistD17. I leave the exact placement of where everything is in the stack to you, but it would be nice to solve #8 and goatcorp/Dalamud#961 at around the same time and with the images included in the zip, so that you can import a zip and see it exactly as you would see it in main repo.

Assigning to goat for now, but he can delegate as required. Might want to break down where things should be if delegated.