For class constructors and `clone', if a keyword argument is specified
multiple times, the latter instance wins, which in our case was the
outdated one.

As a result, it was impossible to change the level of a suffix if that
was set inline, in the prefix definition.

Fixes [1: bcc0bf83].

1: 2025-05-16 bcc0bf836808e156262d9ff631be7d91a0ec201d
   Improve format of layout and handling of included groups

We only started this validation in [1: 45de8e46] and the justification
boiled down to "we might as well".

However these functions are being called by `transient--get-layout',
*before* that had a chance to upgrade from the old to the new format,
and here we perform the validation by calling `transient--get-layout',
which together resulted in an infinite loop, and this errors being
repeated many times:

  Not a legacy group definition: PREFIX: \
  (error "Not a transient prefix command or group definition: PREFIX")

1: 2025-05-16 45de8e46f066eab3160f62cbcc76859022c9c1cc
   transient-parse-suffix: Accept object as prefix

We already call `transient--post-exit', which also takes care of this,
but to be absolutely sure, also reset here and unconditionally (i.e.,
even if `transient--prefix' is nil or `transient--exitp' is `replace').

Before the emergency exit was added, unexpected bugs could look up
Emacs.  That does not happen anymore, and we can also rely on it
to guarantee quitting is enabled again.

Re #388.

Like the somewhat similar but much less complex `read-key-sequence',
for example, Transient menus should inhibit quitting, otherwise C-g
might, for example, kill some subprocess, when the user intended to
exit the menu.

Due to how Transient is implemented, there is nothing around which we
could let-bind `inhibit-quit', like that is usually done.  Instead we
have to set the global value and make sure we reset that when exiting
the menu.

The resetting has to happen on `post-command-hook' but at that time
a let-binding for `inhibit-quit' is in effect, so we have to use
`set-default-toplevel-value', instead of `setq'.

Closes #388.