KBuildSycocaProgressDialog: run kbuildsycoca5 with QProcess.

The dbus-callback stuff from kded is unreliable and unnecessary.

And meanwhile I made kbuildsycoca5 work well when called more
than once, the second instance will wait for the first one
and then check if there's anything else to do, otherwise quit early.

Change-Id: I24f62cf1015bdbab1778245f4221f03f40a9bd0f
BUGS: 235486, 166792
REVIEW: 125314
FIXED-IN: 5.15