muse-project: Fix bug with project file-alist and multiple saved files
* lisp/muse-project.el (muse-project-determine-last-mod): New function
split from muse-project-file-alist.
(muse-project-file-alist): Improve docstring.
(muse-project-add-to-alist): New function that ensures that the current
file is added the its project file-alist.
(muse-project-after-save-hook): Call muse-project-add-to-alist instead
of muse-project-file-alist. This fixes an issue where if several files
were being saved at once, only the first would make it into the project
file-alist.