Turn SourceFormatterController::formatFiles into a job

Summary:
Mass processing of data/files is better done in a job
(ideally one day also in own thread).

Test Plan:
Works as before when e.g. invoked on toplevel project item,
Stopping the job works as well, happens right after the current
file being processed.

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6966