If the file was recognized by a decoder plugin, accept it - don't
require metadata.

Eliminate duplicate code.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Allows forward-declaration.

Reduce heap allocator overhead.

Replaces GLib's GError.

Annoying false positives.

Use _scan() instead, to have more control.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.