baloo-widgets: Refactor filemetadataprovider for better readability

Summary:
Prepare fixing bug 388583
Make signal emission more obvious
Make it easier to distinguish synchronous and asynchronous parts

CCBUG: 388583

Test Plan:
Visual inspection
Make test

Reviewers: elvisangelaccio, ngraham, vhanda, smithjd, #dolphin, #frameworks

Reviewed By: elvisangelaccio

Differential Revision: https://phabricator.kde.org/D10105