Adds SC.Children which allows for marking objects as children of another.

With this API, objects can become owned by another, having the parent
object control the lifetime of the child.

Both property keys and objects can become children, and have slightly
different behavior.

### Property keys as children

- Declared with childProperties or addChildProperty.
- Removed with removeChildProperty.
- When the parent object is created, any child properties that are classes
  will be automatically instantiated, and the resulting object will be set
  as the new value of the property.
- When a new object is set on the child property, the old one will be
  destroyed.
- When the parent object is destroyed, the object at the child property
  will be destroyed and set to null.

### Objects as children

- Declared with addChildObject.
- Removed with removeChildObject.
- When the parent object is destroyed, the child object will be destroyed.
Fix cssPrefix detection on IE11.

[obsoleted-by-master]
Fix handling of drag reorders in SC.CollectionView.

There were some cases where the drop location for a reorder would be
incorrect. In addition, this could lead to a crash in one case when an
index of -1 would be used for the drop location.
Fix SC.TextFieldView hint layout again.

Simply calling _fixupTextLayout on didAppendToDocument was not sufficient,
since the text field could be hidden when it is appended to the document.
Bumping to SC 1.9.0.20.