PCA: retain all data (Unification of embedding methods )

[mstrazar]

Issue: PCA discards the original attributes and replaces them with PCs. Often (e.g. in scOrange), it is desirable to use the original features: annotate PCA plots, compute differentially-occurring features in different PCA regions, etc.

Proposed solution: Place PCs in meta attributes and rename output to Data.

This would be consistent to t-SNE and MDS. The user could use Select columns to restore current behaviour. This is substantially easier (number of PCs is typically small) than merging original attributes with principal components, as it's possible in the current version.

Such an improvement would also play nice with the new widget Apply domain, which enables projecting data to existing PCs and/or attributes.