EnhancedGrid DnD patch for 1.6 beta

[evan]

Delta changes based on #11723, mainly includes following changes:

1. Improved performance by changing avatar style instead of keeping appending/removing it from the document.
   

2. Support dragging content(row/cell) from other widgets into grid.
   

3. Bug fix - keyboard events should be connected to doc rather inside grid as row/cell can be dragged out of grid.
   

4. Bug fix - the avatar mark was not correct in RTL.
   

5. Bug fix - occasional conflict with selection
   

6. DnD styles moved into a separate "DnD.css"
   

7. Code clean, removed unused logic
   

8. Test case/data updates

[Douglas Hays]

(In [23322]) Fixes #12065, refs #11723. Proxy commit for evan (IBM, CCLA). Various EnhancedGrid? DnD bug fixes and new tests. !strict

[Evan]

(In [23617]) Refs #12065 Remove the obsolete empty dnd folder

[evan]

(In [23641]) Refs #12065 1. remove redundant code 2. don't rearrange store content for id column

[evan]

(In [23677]) Refs #12065, fix a variable typo which caused failure of dnd content among Grid instances

[evan]

(In [23790]) Fix from Xiao Wen Zhu(IBM, CCLA), fix a compatibility issue of DnD and disabled rows, refs #12065, !strict

[evan]

(In [23818]) Trivial func naming & test case fixes, refs #12065

[evan]

(In [24279]) Refs #12065 removing 1 obsolete line