Restore keywords when restoring last browser session

[ator-dev]

This feature requires that MMS can "remember" the keywords from the tabs in the last session when the browser is reopened, and restore the ones that are still relevant.

This will likely be implemented (if at all) as a plugin, since it will need specific code to overcome these obstacles:

• Tab IDs are not persisted between sessions
• Keyword storage is automatically cleared to save on memory/storage (depending on the browser, since currently Firefox does not implement session storage)