Refactoring to comply with changes in 4.0

[NiclasEriksen]

These are breaking changes and shouldn't be merged into master yet, but these are sufficient to test and use this plugin under 4.0.

Tested on Alpha 6, no issues detected and seems to behave just the same as on 3.x.

[lawnjelly]

Yes I guess we should move to having a 3.x branch and 4.x. Good that it seems not many changes are needed.

Hopefully this will be temporary as there should be core interpolation in 4.x in the future, based on the feedback from the core interpolation in 3.x.