Enable sort on multiple columns

[severo]

• change the behavior of the clicks on the column headers: the clicked column becomes the first column in OrderBy, with direction ascending, if it was not the first one. If it was the first one, cycle through: ascending -> descending -> none.
• process any number of column sorts in sortableDataFrame (instead of 0 or 1 before): sort along the first column, then use the second one in case of ties, etc.
• handle multiple columns in OrderBy in rows selection

cc @platypii: there are numerous TODOs in the code because there are plenty of opportunities to optimize (ie. to avoid doing the same again and again). I think we can work on optimizing in another PR (and I can better move these TODOs to a specific issue), though, because it could require changing a bit the format of DataFrame. Interested in your thoughts on how we could optimize.

[platypii]

very nice!