Plugin System Epic

[xen2]

Xenko needs to be reorganized so that Physics, Video, SpriteStudio, etc... should become separate optional packages (referenced explicitely by games).

Also, we would like to be able to more easily create custom asset types and have a proper Game Studio integration (associated importers, editors, thumbnails, previews, etc.)

Doing this work will make Xenko much more extensible in general for people who want to add their own features externally.

It should also make the runtime leaner and better organized (i.e. Video playback currently forces to embed tens of megabytes of native DLL even if the game doesn't use it).

Part of this work is needed for #12

[jmkinzer]

My $0.02: #12 + #13 seem the most important items to address in order to (1) allow new features to be added cleanly (2) hopefully thus also encourage contributions and (3) get the likely most extensive breaking changes over early-on.

Inevitably serious projects seem to wind up with a big-ball-of-mud "engine module" that over time dwarfs and references everything else. For example the two major C++ engines one can easily name fall under this category.