Reviewed-by: ck

Access to the users private resource path (derived from the
settings path) has been used in quite some places, so it's
worth moving it to the core interface centrally.

Reviewed-By: Thorbjorn Lindeijer

Reviewed-by: ck

 - lessens open file handles
 - improves help plugin startup further

Task-number: QTCREATORBUG-337
Reviewed-by: ck