Qbs: Simplify and fix qbs node management

The code is simpler now and also fixes a reported bug.

Task-number: QTCREATORBUG-9378
Change-Id: Ibb8cd7b074e840eb413b8d433405e47dcc63ee06
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Qbs: Improve display of absolute directories in Projects view

Change-Id: I89c7764f5eaa066315a00f8d3c38df3d338c2ab3
Reviewed-by: Daniel Teske <daniel.teske@digia.com>