moving figured bass properties to `Score` context

[Werner LEMBERG]

Right now, only

```
figuredBassFormatter = #format-bass-figure
```

is set in the definition of `Score` (in file `engraver-init.ly`).
Listening to Jean I think it's best to move all other figured bass
context properties to that location, too.  These properties are

  figuredBassLargeNumberAlignment (default: CENTER)
  figuredBassAlterationDirection (default: #f)
  figuredBassPlusDirection (default: #f)

There will be one more (`figuredBassPlusStrokedAlist`) that I need for
my new figured bass symbols in MR 1460...

WDYT?


    Werner