Improved XML inline-related translation features

[k-yaegashi]

Resolves the issues #389 and #391.

This fix can

• treat placeholder for empty elements, and
• translate attributes inside attribute-folded inline and placeholder.

In order to realize former feature,

• extracted special processes for open and close tags in treat_content and put into treat_content_open_tag and treat_content_close_tag respectively,
• applied both treat_content_open_tag and treat_content_close_tag for empty-element tag in treat_content, and
• disabled tag breaking for placeholder-marked empty-element tag in tag_break_alone.

In order to realize latter feature, when processing open tag marked as placeholder or attribute-folded, passed the tag into treat_attributes.

[mquinson]

Hello @k-yaegashi,

thanks for these changes, they seem very good. I hope to see more such changes in the future ;)

It would be easier for me if you could split your changes into several commits. Here for example, it'd be better to have one commit splitting the functions apart, and another commit changing this code to implement the new feature.

Another thing that would be really welcome the next time that you commit to this project (soon, I hope) is that any new feature must be tested accordingly. This one should be easy for you since you already have the test files in the bug reports (which are very good, thanks for that).

Again, I don't want to criticize your work. It's only that maintaining po4a is unfortunately very low priority for me right now and I desperately need help from new contributors like you to keep it going. Thanks again for this change, and I'm waiting for the next one ;)