Fix 3D views sharing viewport engine after splitting/duplicating

Some UI changes for new viewport engine dropdown

* Use legacy viewport as default for now
* Remove "Use Modern Viewport" option
* Minor cleanup

Implement generic draw-modes and draw-plates for viewport engines

Also made the new viewport use these (at least a default mode with a number of default plates).

This draw-plate design is more generic then the previous hardcoded one, it stores the plates in mutable lists which can be stored as presets (draw-modes). We can further add sub-plates and process callbacks for plates that need additional data processing or don't do any drawing.

Use viewport engine for drawing new viewport

Only basic transition, objects are not drawn yet.

Initial support for generic viewport render engines

Adds a basic API for managing viewport render engines. This should make it possible to hook up external viewport engines in future (e.g. Hydra). Commit also includes some basic integration with View3D (including version patching) and RNA defines for exposing viewport render engine list in UI.

Code works really similar to normal render engine API, but I don't think we should merge them.