[DataGrid] Rename sortModel to sortBy

[oliviertassinari]

This change aims to make the objective of the prop easier to grab and memorize. It's pretty similar to what most ORMs have, matching the ORDER BY command of SQL. First discussed in #130 (comment).

[dtassone]

For this issue, we should also consider the corresponding event names.
For sorting, we currently have sortModel and onSortModelChange
What would be the event name if we rename sortModel to sortBy?

[oliviertassinari]

What would be the event name if we rename sortModel to sortBy?

To match the conversion of exposed states: onSortByChange?

[dtassone]

It literally doesn't sound nice in english I think.

How about onSortingChange, onGroupingChange, onFilteringChange or onSortChange, onGroupChange, or onFilterChange...
versus onGroupByChange or onFilterByChange

[oliviertassinari]

It literally doesn't sound nice in english I think.

Time to bring in @mbrookes 👀 😄.

IMHO, on this subject, it doesn't matter if it sounds English or not, the convention is to take the name of the state, and to use onXChange, defaultX to complement x.