Register file formats using decorator

Signed-off-by: Michal Čihař <michal@cihar.com>
Sort supported file formats

Signed-off-by: Michal Čihař <michal@cihar.com>
Early validate file format

Signed-off-by: Michal Čihař <michal@cihar.com>
Check for file format support when starting Weblate

This way we offer only valid choices in the interface.

See also issue #491

Signed-off-by: Michal Čihař <michal@cihar.com>