Re: Add -other-{window, frame} variants of project-prefix-map commands

[Juri Linkov]

> Note also that the tension between parsing `C-x 4 FOO` as "lookup FOO in
> `ctl-x-4-map` (or in `ofw-transient-map`)" and "lookup FOO in the normal
> way", which is why in `ofw--set-prefix` we do:
>
>  ;; FIXME: Setup a transient map to reproduce the behavior of the `C-x 4/5'
>  ;; prefix keys, but only do it in "regular" buffers.  In special buffers,
>  ;; these extra bindings may be more annoying than anything, e.g. in Gnus's
>  ;; summary buffer `a' open a new message buffer, so we'd want `C-x
>  ;; 5 a' to open it in a new frame, but instead of calling
>  ;; add-change-log-entry.

The latter ("lookup FOO in the normal way") is handled now by `C-x 5 5 a`,
so the transient map for the former ("lookup FOO in `ctl-x-4-map`")
could contain only cherry-picked shorthands to reduce longer key sequences
like `C-x 5 5 C-x C-f` to just `C-x 5 f`.

Maybe even it could employ some heuristic to automatically deduce
the transient map based on the existing `C-x` keymap.  For example,
from `C-x p b` (project-switch-to-buffer) it could build a new key
sequence `C-x 5 p b`, etc.  This could help to close bug#42210.