Refactor PO section marking

Use msgids to mark sections. In the PO format, comments are associated
with specific messages, which does not match the semantics for section
markers.
Furthermore, comments are not preserved by `msgmerge`, which required
quite convoluted handling to copy them over from the template.
By using msgids to mark sections, this problem is avoided.

This convoluted handling was also used for header comments. Header
comments are now handled in a simpler way. There is a fixed prefix,
identifying these comments, as well as a list variable containing the
lines which should be put into the header. When a PO file is generated,
all existing lines starting with the prefix are deleted, the and the
current version of the lines is prepended to the file.

Closes #11845

build_tools/fish_xgettext.fish

@@ -31,7 +31,15 @@ begin
         or exit 1
     end
 
-    echo '# fish-section-tier1-from-rust'
+    function mark_section
+        set -l section_name $argv[1]
+        echo 'msgid "fish-section-'$section_name'"'
+        echo 'msgstr ""'
+        echo ''
+    end
+
+    mark_section tier1-from-rust
+
     # Get rid of duplicates and sort.
     msguniq --no-wrap --strict --sort-output $rust_extraction_file
     or exit 1
@@ -77,13 +85,13 @@ begin
         # This regex handles explicit requests to translate a message. These are more important to translate
         # than messages which should be implicitly translated.
         set -l explicit_regex '.*\( *_ (([\'"]).+?(?<!\\\\)\\2) *\).*'
-        echo "# fish-section-$tier-from-script-explicitly-added"
+        mark_section "$tier-from-script-explicitly-added"
         extract_fish_script_messages_impl $explicit_regex $argv
 
         # This regex handles descriptions for `complete` and `function` statements. These messages are not
         # particularly important to translate. Hence the "implicit" label.
         set -l implicit_regex '^(?:\s|and |or )*(?:complete|function).*? (?:-d|--description) (([\'"]).+?(?<!\\\\)\\2).*'
-        echo "# fish-section-$tier-from-script-implicitly-added"
+        mark_section "$tier-from-script-implicitly-added"
         extract_fish_script_messages_impl $implicit_regex $argv
     end