Eliminate the global variable "shutdown_loop".

Fix for a major memory leak.

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.

If fs_charset is empty, i.e. we're using the default "utf-8",
GetFSCharset() should return exactly that instead of an empty
std::string.

Implement a fast path for UTF-8 which leaves fs_charset empty, and
don't assign a value to fs_charset if there's no configuration.

Add UpdateQueueItem::id to keep track of the id in every item.
Replaces thhe hack in update_queue_push().

Fixes a few kludges and avoids GLib allocation.

Decouple some more from GLib.

Allows forward-declaration.