TMX Upload working from web.

Also refactor the hibernate mappings for some of the TM related entities.
Refactor TM model.

Trans unit variants are now independent for each Translation unit.
Block the UI while an upload is going on.
Add a "clear" option to clear all the contents of a Translation memory.
Merge existing TUs when importing a TMX file.

This means that existing TUs will be replaced with new ones as long as their unique id is the same. Otherwise, the new TUs will just get appended to the existing ones.
Add smaller tmx test files.