Decimate with Shapekeys support

[Haggets]

This adds proper support for decimation with shapekeys through a modifier handler that is applied after the standard process. It uses an decimate operator instead of the modifier, which works identically but keeps the shapekeys without breaking them.

The only caveat is that it only works with collapse, so unsubdivide and planar decimation are not supported since there is no operator for them as far as i'm aware.

Thanks greisane for this great plugin! Would be nice to see you post content on Twitter again!

I've never merged before so let me know if i messed something up.