This prevents issues like #1731 and is an (better) alternative to #1732.

(@mrmr1993: This is yet another approach to the Settings problem.)

With the new Settings implemetation, settings which have a non-default
value and which are not in synced storage (that is, they have not been
changed since synced storage was introduced) are not currently
accessible to content scripts.  This commit makes such settings
accessible via chrome.storage.local.

Important:
- There's a change to the established settings data model here.
  Previously, settings with default values were not stored; here, they
  are.  This eliminates a considerable amount logic from Settings, but
  means that migrations will be required if default values are changed
  in future.  (Other than type changes, have we ever changed a default
  value?)
- There's also a change (bug fix?) to the behaviour when an affected
  setting is reset to its default value.  Previously, the change would
  *not* have been synced (whereas all other changes are).  Here, such
  changes *are* synced.  The previous behaviour was inconsistent with
  the syncing behaviour of all other options changes.

Note:
- This isn't particularly well tested.  It's being committed mainly just
  for consideration of the approach, initially.

(Not sure when this crept in.)

We need to ensure that we always generate the same hint strings for the
same filter state.  Here, we do this by always using the same mechanism
(@filterLinkHints) to set the hint strings.

Functionality wise, this is a no-op.  However, since we now create a new
LinkHintsMode object on every "f", we don't need to be so careful about
resetting the state on exit.

LinkHints.init() isn't doing anything.

This is a minor re-working of #1705 from @mrmr1993.

The main changes are:
   - Simplify initialisation logic.
   - Always initialise Settings immediately and automatically (ie. don't
     initialise Settings separately and manually in the background,
     content scripts, options and tests).
   - Get rid of addEventListener (it's only being used for on "load").
   - Add Settings.use() in its place.