[IMP] partner_relations: merge from development

This branch fixes two functional issues
- better check for overlapping relations
- edit relations within a popup

And improves some poorly phrased dutch translations
Fields with really one attribute, that is only the name, I prefer on one line. When there are multiple attributes I prefer multiple lines, first to keep the line length within limits, secondly for easier adding, copying and deleting attributes. It certainly helps when using line oriented editors like vim.