Move Data Transformer's functionality into connections

[soininen]

Data transformer is quite confusing to use as it doesn't do anything in itself --- it merely modifies database URLs such that the next item to "sees" the transformations. I think moving the functionality into the connections between items would be more logical.

In broad terms, the changes would be:

• Add "Data Transformations" settings to Link properties tab, enabled when there are filterable database resources available. The setting would be modified via Data Transformer's specification editor.
• Move the functionality from Data Transformer's executable item into Connection class. The temporary files generated by the item could be stored into <project root>/.spinetoolbox/local/tmp/ or similar as there we don't have connection specific data directories.
• Deprecate Data Transformer.

[jkiviluo]

Not sure about this. While this would be understandable in current situation, we do have a need for a more capable data transformer that performs operations either between DBs or between importer and DB or between DB and exporter.