The `SpecialFolderType` was somewhat mis-named, since the special use
does not (in most cases) confer any special type, rather it's simply
defines what a particular folder is used /for/.
As such, rename the enum to to `Geary.Folder.SpecialUse`, moving it in
to the `Folder` class, since it relates specifically to folders, also
rename `Folder::special_folder_type` and
`Folder::special_folder_type_changed` reflect the above and not
duplicate the type name, and similarly rename
`Account::folders_special_type`.
Update the many call sites.
Remove functions that have as-convenient methods in up-tp-date Gee,
rename ::get_first to ::first so it reads better and add API docs,
replace ::to_array_list with ::copy, since that's what's actually
needed.
Add a ContactHarvester arg to ImapDB.Folder.create_or_merge_email and
call it on the resulting email. Add and update a new harvester
property to MinimalFolder and pass that in as needed.
Replaces harvesting code in ImapDB code so as to be stand-alone and
reusable. Implement harvesting policy to only harvest from a set of
whitelisted special folder types (so junk and trash is not harvested)
and only harvest valid, non-spoofed, addresses.
