Declutter the client and client manager by moving find usage and follow
symbol into this helper. This functionality was "temporary" put into the
client manager, but is more an implementation detail that shouldn't be
handled in that central place. Rename symbol will also go into this
helper class.
Task-number: QTCREATORBUG-21578
Change-Id: I56680f6ccbb8d244066561167178af7b341b8822
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
