QmlDesigner.instances: Always use distance field text rendering

This avoids sub pixel antialias artefacts.

We call disableNativeTextRendering() in updateDirtyNodesRecursive(), because
items might be created or change the property after component complete.

Change-Id: Icd9c87b69434a910c9555540ba7bafdc874afc5a
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>