Bump release to 0.13, move images to skin directory, fix extension ID.
filelink: Allow to insert grants directly from Thunderbird.

- Use the NotYetAvailable(tm) /newgrant REST API.
- Extent the nsDL component with a new interface to allow for new grant
  create/delete/url methods.
- Overlay the message composer to add a new toolbar item and a new tool menu.
- Automatically show the filelink "add new account" dialog if no existing DL
  account can be found.