Usurp more of the utils code than can be re-used without further dependencies in
helpers.
Split off specific parts in their own file.
After the helper/utils changes, update the rest of mu4e to take the changes into
account.
Use "_" as the title of that section so that it is less distracting
when sections are collapsed to get an overview of the library.
Using a separate section is useful because it reduces the risk of
accidentally into the middle of a library.
Placing two semicolons on an otherwise empty line helps to logically
"connect" the surrounding "paragraphs", which in (only) some cases
makes sense.
Previously the three paragraphs of the permission statement were not
connected to each other like this, which is perfectly fine. However
the preceding "This file is not part of GNU Emacs." line was connected
to the first paragraph, which does not make sense considering that the
latter is not connected two the second paragraph, which it relates to
more.
Once those two semicolons are gone, it also makes sense to remove
those from the second line.
Make the structures use for mu4e-bookmarks a defstruct, and update its
usage throughout the codebase. This makes it a bit easier to read and
extend.
Ensure that the old-style bookmarks are automatically converted.
