Version 3.0

This feature branch should prepare version 3.0 of XML Importer. It should fix some of the extension's major issues – I just need to convince @brendo to work on this with me :)

### Features

#### API – @brendo 

- [ ] change XML import from direct sources to Symphony Data Sources
- [ ] make sure all validation errors a stored and prompted to the user
- [ ] if the import failed, only show entries with errors and not all entries (also reflect this in the error message which currently states `1234 entries of 1234 entries did not validate` although it might just have been 1 or 2)
- [ ] if the import failed, allow the user to ignore errors and only import the valid entries

#### UI – @nilshoerrmann

- [ ] change import UI from direct links to Data Sources
- [ ] make the error page easier to read, giving better feedback