Allow headers only at the start of a document
Revise and extend header scopes
Cleanup comment types and scopes
Add match for obsolete entries
Don't mark empty lines as invalid
Revamp item matches in grammar

Cleanup scoping and support multi-line comments, fixes #1. Support more than one file reference in `comment.line.number-sign.reference`.
Simplify flag match

#ignore
Ignore non-empty multiline strings in next/previous macros

Fixes #2, thanks to Martin Häcker. Also add support for plural forms of `msgstr`.