Improve sorting

[wyuenho]

Sorting is problematic for a number of reasons:

• No obvious way to set default sorting (Move _direction state variable out of HeaderCell and into Column #256, Sorting questions. #222)
• No obvious way to sort programmatically
• Can't sort by multiple columns ([Feature Request] Sort by multiple columns #143)
• Can't sort if the header is disabled (Make header optional (like footer) when initializing Grid #233)
• HeaderCell is doing too many things too inefficiently.

I really need a long day to fix these issues once and for all. 😫

[kriswill]

+1 -There may be many breaking interfaces with these things...

[wyuenho]

Yeah well, if you have a customized HeaderCell, prepare for some major breakage...

[kriswill]

Yes, I do...and that's OK. I'm fully prepared to absorb changes.

[wyuenho]

Closing this. Moving multiple column sort to 0.4