Add AS_NODE_INSERT_FLAG_MARK_TRANSLATABLE

This simply adds an understore to the tag name which marks it as translatable
for intltool.
Add as_node_find_with_attribute()

This allows us to find a tag with a specific attribute.
Add AS_NODE_TO_XML_FLAG_SORT_CHILDREN

We may wish to sort the tags in the file when outputting human-readable XML.
Include a newline before XML comments to help them stand out