Rename Transform widget to something more explicit

[markotoplak]

Perhaps "Apply preprocessor" or something as descriptive could be clearer.

Originally idea from @borondics.

[janezd]

I started renaming, but then I realized that "Apply preprocessor" is not a really good name since it implies that the input must be some kind of preprocessor (well, the widget already indicates so...) in a "narrow sense". This may be misleading; preprocessor would often come from widgets that compute, for instance, some projection, which many users won't associate with preprocessing.

I support the idea, but we should look for a better name.

[mstrazar]

Transform
Apply Preprocessor
Apply Transform
Embed
Project
Back-project

[janezd]

Unclear name is caused by the ambiguity about what this widget actually does; see #3623.

[janezd]

Fixed in #3673.