bug#66317: Project mode-line

[Juri Linkov]

> This looks nice (I actually have something similar in my
> `mode-line-format` already), but I think that modifying
> `mode-line-format` when project.el is loaded is a bit problematic.
> Perhaps a better alternative would be to define a variable that holds
> this mode-line construct, along with an autoloaded function that people
> can call from their init files to actually add this construct to
> `mode-line-format`.  WDYT?

In bug#63469 I implemented 'project-mode' with the buffer-local
variable 'project-name'.  But due to objections I rewrote this
to simpler and direct implementation.  I could revive the previous
patch now.