gkeyfile: Fix group comment management

This removes the `comment` member of the GKeyFileGroup structure, which
seemed intended to distinguish comments just above a group from comments
above them, separated by one or more blank lines. Indeed:
* This does not seem to match any specification in the documentation,
  where blank lines and lines starting with `#` are indiscriminately
  considered comments. In particular, no distinction is made between the
  comment above the first group and the comment at the beginning of the
  file.
* This distinction was only half implemented, resulting in confusion
  between comment above a group and comment at the end of the preceding
  group.

Instead, the same logic is used for groups as for keys: the comment
above a group is simply the sequence of key-value pairs of the preceding
group where the key is null, starting from the bottom.

The addition of a blank line above groups when writing, involved in
bugs #104 and #2927, is kept, but:
* It is now added as a comment as soon as the group is added (since a
  blank line is considered a comment), so that
  `g_key_file_get_comment()` returns the correct result right away.
* It is always added if comments are not kept.
* Otherwise it is only added if the group is newly created (not present
  in the original data), in order to really keep comments (existing and
  not existing).

Closes: #104, #2927

0 files changed, 0 insertions, 0 deletions