Extract the compile functionality into an API function

[bartfeenstra]

Catalogs can currently only be compiled using the command line. To allow applications to leverage Babel to compile translations themselves, the relevant code can be extracted from babel.messages.frontend.compile_catalog() into its own API function, separating it from the command line.

Right now this has to be done by invoking the Babel command line, like in bartfeenstra/betty#619

[akx]

PR welcome!

[azmeuk]

PR welcome!

@akx Have you expectations on how this should be done? Just extracting the logic from the Command object in babel/messages/frontend.py is enough? Should the PR also do this for all the other commands in the file?