Modeled after std::filesystem::operator/() from C++17.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Type-safe wrapper for uri_get_suffix().

Move code from Path::RelativeFS() and make it generic.

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.

Move code from map_fs_to_utf8().

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Migrate the remaining callers to FatalError().

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.