Change tmx import to be able to use html multipart fileupload

Add a second alternative for the tmx import: it should be possible to use multipart file upload of a  plain tmx file (not base64-encoded) to t5memory for the import.

Or alternatively it should be possible to send the tmx as base64-encoded zip-file to t5memory.

What ever is faster to implement. If it does not matter, choose the stream.

Coordinate the development time needed and therefore the way to go with Orest. It will be mainly his development time needed that decides on the way of implementation.