Non-nested minibuffers (was: Stop frames stealing eachothers' minibuffers!)

[Stefan Monnier]

> !m_f_s_f is that a minibuffer is attached to a frame.  So actions in a
> different frame shouldn't affect it.

Notes that in practice they do anyway to some extent because minibuffer
sessions are *nested*: you can't enter minibuffer 1, then enter
minibuffer 2, then exit minibuffer 1, then exit minibuffer 2 (if the
users try to do that, when they try to exit minibuffer they should
either get an error  1 or have the minibuffer 2 automatically aborted
before we actually exit from minibuffer 1).

It might be interesting to try and use ELisp threads to allow non-nested
minibuffer sessions.  
I mean, if you're interested in opening that box ;-)


        Stefan