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