This refactoring is based on Daniel Mensinger's idea detailed in
kdevelop/kdevelop!45 (comment 12455)

The added class FileFormatter gets rid of repeated parsing of formatter
and style config entries. The added interface IFileFormatter simplifies
non-source-formating code that used
ISourceFormatterController::formatterForUrl(); such code no longer
depends on ISourceFormatter.

Other improvements:
* Reduce the number of QMimeDatabase::mimeTypeForUrl() calls.
* Improve SourceFormatterController's documentation.
* Deduplicate SourceFormatterController's config-related code.
* Optimize the types of constants in Strings namespace.
* Don't waste time parsing formatter configuration if formatPolicy
  equals DocumentChangeSet::NoAutoFormat.