Add missing copyright header.
PEP8 complient output for latexmap.py.
Correctly return error codes

Not all commands passed the return value of the worker function
back up. Not all of them can return errors, but this makes it
more consistent.
Also all worker functions now save the survey before returning
(reorder command was different). This makes the behaviour more
constistent at least.
gui: print warning when no images have been added.
gui: Fix backtrace when toggling a sheet to be valid.
Remove .pot file. It is updated automatically every once in a while.