Tags are not kept sorted alphabetically in sidebar

[nekohayo]

In the current git master version, since the port to GTK4, tags are not sorted alphabetically, ever.

• Newly created tags (ex: by typing a whole new tag in the contents of a task, then closing the task editor) are not inserted sorted alphabetically into the sidebar
• Even after a restart, tags in the sidebar remain in the order of creation, rather than a more natural alphabetical order

The 2nd point tells me that they are surely not sorted within the tags list at the top of the file format either. That said, perhaps the sorting should be done only at the UI level, and kept manual in the file format, so that it is technically possible someday to toggle alphabetical-vs-manual sorting on/off at the GUI level for #523.

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Note: when fixing this, the tag's hierarchy levels need to be taken into account, because tags can be nested as parents vs children (see also #1249 for example).

So if I have tags like these:

• a
• c
  • d
  • e
  • f
• g

…and I insert tag "b", if it's a top-level tag it would go here:

• a
• b ← new tag!
• c
  • d
  • e
  • f
• g

…but if I make it a child of "c", then it would look like this:

• a
• c
  • b ← new tag!
  • d
  • e
  • f
• g