<choice>: output first child in main text

[lueck]

The handling of <choice> should be generalized. The first child should go into the main text; the other children only into the apparatus.

This way we can write general code while leaving up to the encoder to decide if, e.g., a <corr> or a <sic> shall go into the main text by using the order of the children.

This is possibly a breaking change!

• first child goes into main text
• apparatus has lemma made up from first child
• apparatus has subsequent children as readings
• static text in the apparatus for certain elements
• regression test for ALEA