Does anyone know of an extension or other way to allow me to save a set of files in a project for later use?

Example: - Working on site A, opened html files A1-A15 (15 files)

• Received a request to work on site B, new files (number unimportant).
• I would like DW to remember that I was working on files A1-A15.
• Close the site A files and focus on just files from site B.
• Complete site B work.
• Reopen site A files altogether.

Suggestions are greatly appreciated.

Thanks!