making OMF to provide sorting at core level (OmfDb::listObjectsBy)

[christiansalazar]

Sort must come available as an extra argument to listObjectsBy, in a form of a key-pair array, so the desired property names to be sorted for must be mapped to the real storage and let it to do the magic (to sort by those properties).

YiiOmfDataProvider will start sorting after that change without a little effort: passing the selected sort key fields to listObjectsBy and magic comes in.