baloo-widgets: Emit metaDataRequestFinished once per request

Summary:
Add signal 'dataAvailable' to allow chunkwise data processing
Emit metaDataRequestFinished when request processing is finished
Adapted execution order to ensure loadingFinished is signalled last

CCBUG: 388583

Test Plan:
Visual inspection
make test

Reviewers: elvisangelaccio, smithjd, vhanda, ngraham, Dolphin, Frameworks

Reviewed By: elvisangelaccio, ngraham, Dolphin

Subscribers: dhaumann

Differential Revision: https://phabricator.kde.org/D10113