[perl #108470] Make Term::ReadLine use AE if available

[perl #108470] Make Term::ReadLine use AE if available

Term::ReadLine only allows the Tk event loop to be called during
a readline call. This should be updated to use AnyEvent which will
still work with Tk, as well as any other event loop the user may need.

With this patch, T::RL now uses AnyEvent if it is loaded, falling back
to Tk otherwise; so the Tk mode won't be affected.

T::RL::Stub has its own get_line. This does not honour the tkRunning
flag at all. If I remove it, it's fine. This patch does so.