NautilusPropertyDialog: Fix problems with fast group/owner changes

If the owner or group of a file was changed twice very quicky then
the second change would cancel the first. However, the code assumed
that after the nautilus_file_cancel() call the result of the first
change would not be called. That is not true anymore, and when
it did get called it tried to stop the timed wait, which gave a warning,
and in general touched stuff that the new operation should only touch.

We fix this by making A group/owner change its own struct which we
delay freeing until the operation has actually finished, although we
track if it has been cancelled so we can avoid multiple calls to
eel_timed_wait_stop().
nautilus_file_get_owner_name: Don't ever return "me"

This function is used by the property window code to compare the actual
username to one from a function, so don't use "me" unless we're purely
for display purposes.
NautilusPropertyDialog: Fix user list for owner change

This code was constantly failing to find the current files user as
the form of the "owner" attribute didn't match the internal list.
Also, as a consequence of this the user list didn't match the old
one so we keep re-creating it all the time.

We fix this by being much more explicit in what we store in the
model columns:
0 - the display name
1 - the return value from nautilus_get_user_names()
2 - the actual username (not real name)