General bugfix
Introducing FileArchive table

And adding default three archives :arch0:, :trash: and :sandbox: on
init.
ArchiveManagerFactory
FileManager::PopulateAndRegisterFileArchives reads archives from DB

Also changes name of URL field to Path in FileArchive table in DB.
Bugfix, decompose FileManager::InitializeFileArchives

to PopulateAndRegisterFileArchives and DoFileArchiveInitializations
Updated `Config` field names and constantized sandbox

Used constant for sandbox and trash everywhere, and for :arch0: in
places where it was not dynamic.
Introduced Config::currentFileArchiveForAddingFiles

We can have a UI to change it, but anyway.
Replaced occurences of :archive: with:
constant conf->fileArchiveNamePATTERN.arg(X) wherever it was constant,
or :arch0: in an explicit default value,
or conf->currentFileArchiveForAddingFiles in BMEditDialog for adding new
files,
or :ArchiveName: when there was general speaking about archives.
END FileArchiveManager

Removed the remaining to-dos. Lesson: If you choose to do small commits,
do NOT BEGIN/END them, as many other, unrelated, things are also fixed
in the way between them. Maybe categorize the commits, but don't
begin/end them.