QmlDesigner.PropertyEditor: Allow more space for labels.

On some systems the fonts are eally huge, so we have
to allow much more space for the leading label.

Task-number: QTCREATORBUG-12106
Change-Id: I7ad6687c4af15b2126efe2c3bd8284d36d9ac633
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>