*** Improved directory selection in 'project-forget-projects-under'.

This command now prompts using a directory list limited to those in the
remembered project list.  Previously, it would prompt using general
directories from the file system.

I hope this isn't controversial...it made limited sense to me that the
prompt candidates had no relationship to remembered projects.

*** New command 'project-forget-projects-under-dirs'.

This command prompts for multiple directories under which to forget
remembered projects.  The candidate directory list is limited to those
in the remembered project list.

-Stephane