QmlDesigner: Fix preview rendering

We excluded item which had a node instance. For the preview all item should
be marked dirty.

Change-Id: I0cbad89707d7d94f65ac0d055488e124da40e7b3
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner: Better componentComplete

Change-Id: I2bfdbeabd4d08474d50434d7e13c32b780e783ca
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>