Toolkit: Download Manager is now Toolkit: Downloads API

At Paolo Amadini's, who is the triage lead for the component in Bugzilla,
request, I've renamed Toolkit: Download Manager to Toolkit: Downloads API.

The new description for the component is:

For issues relating to the process of downloading files to disk,
including for example progress calculation and timing, folder choice,
file naming, and file system access.

Does NOT cover:
 - The user interface (see Firefox :: Downloads Panel)
 - Networking issues (see Core :: Networking)
 - Choosing the handler application (see Firefox :: File Handling)
 - Saving complete web pages (see Firefox :: File Handling)

The name change is to make the component's purpose clearer, and reduce
the need to change a bug's component.

The triage tool's been update to reflect the change.

-- Emma