Adds layoutDirection property to SC.ListView. Refactors SC.ListView and SC.CollectionRowDelegate to move away from height specific methods and properties to generic height or width methods and properties.

Also fixes issue that rowSpacing set on the row delegate had no effect on the list item layout.
Renames contentIndexRowHeight to contentIndexRowSize.
Updates the documentation to improve grammar and include both vertical and horizontal layout functionality.
Privatizes the convenience computed property of rowSize + rowPadding, because it just adds confusion on which is correct: rowSize vs. totalRowSize. Renames rowSizePlusPadding to _sc_totalRowSize (private).