Geary.SpecialFolderType: Rename class and related properties, signals

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.
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