Set SC.DEFAULT_CURSOR to 'default' instead of 'auto'

Because `SC.DEFAULT_CURSOR` was set to `auto`, all the `SC.LabelView` (or any other view containing text)  inside an `SC.SplitView` (which use the `SC.Cursor` class) were having a `text` cursor. 

With this PR only the views that have isTextSelectable set to yes get a `text` cursor.

Note that I removed `SC.SYSTEM_CURSOR` which wasn't used anywhere and which is now replaced by `SC.DEFAULT_CURSOR`.