automatic type conversion for property assignments

When assigning a string to a stringList property we automatically
convert the JS variable to an Array of strings.
Similar conversions are implemented for Bool, Number and String.

Task-number: QBS-38

Change-Id: I19f54f83e540b20f18ca505d8cd8011735d6418d
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>