Properly indent <Vertex> and <Face> nodes
More performance optimizations.

 - Cache values of len(...) that do not change
 - Do not build the list of UV maps for every face of the mesh, but only
   once per mesh
 - Inline the can_merge_vertices function
 - Do more early-abort tests in the mesh optimization routine before
   calculating actual vertex distances
 - Do not compute angle bisector of two normal vertices during vertex
   merge if the two vertices already share one normal vector
Add printing of callers to commented-out profiling code
Performance: Use tuples instead of lists, fewer max()/sqrt() calls

 - Use tuples when possible
 - Calculate only the square of the bounding radius until the very end
   and do a simple comparison instead of a call to max()
Wrap user-visible strings in _() for localization
Update user-visible texts from current Zusi translation
Add translation template file and German translation
Translate remaining strings and load German translation by default
Update README to reflect translation
Fix tests and always write animation names in German
Remove unnecessary print statement
Make export and import tests pass on Windows
Fix idname properties of export/import operators
Make paths relative to the Zusi data directory if possible
Fix i18n of 'Successfully exported %d files' message
Display list of files to be overwritten when exporting animations
Make the tests pass again
Animation export: Apply all transformations up to the current file's root object
Display list of overwritten files only when it's nonempty
Warn on export if no author information was entered