bug#66317: Project mode-line

[Spencer Baugh]

One interesting bug: project-switch-project causes the modeline project
name to change for all buffers.

For example if I do:

C-x p p emacs-29 RET f

then while I'm at the find-file prompt, all modelines for all buffers
show emacs-29 as the current project, no matter what they showed before
(even if they didn't show anything before)

Haven't thought at all about how to fix this, but we might want to think
about it in the context of bug#63648 where we've been rethinking the
implementation of project-switch-project.