Clean up and homogenize scripts code

### Part 1

- [x] Split scripts into load, convert and store steps. Name them all "px:foo.convert" and not "px:foo-convert".
- [ ] Homogenize names of ports of component steps. E.g. "source.fileset" and "source.in-memory" and not "fileset.in" and "in-memory.in".
- [ ] Possibly convert single-document ports and even anyFileURI options to fileset + in-memory ports.
- [ ] Merge fileset and in-memory ports into a single port, where the first document would be the fileset and the rest the in-memory documents.

### Part 2

- [ ] Add input validation to all scripts (to load steps): https://github.com/daisy/pipeline-scripts/issues/112. There should be an option named "validation" with possible values "off", "report" and "abort" (see https://github.com/daisy/pipeline-scripts/issues/111).
- [ ] Give all scripts a "status" output port with a `d:status` document (for now `d:validation-status`, see https://github.com/daisy/pipeline-framework/issues/121).
- [ ] Give all store steps an output port with a `c:result` document.
- [ ] Make sure all scripts have media-type set on all inputs, outputs and options (https://github.com/daisy/pipeline-tasks/issues/105)