Add loadData support for Nested Rows.

[jansiegel]

Description

Issue #3914 pointed to a problem with loadData not being covered by the Nested Rows plugin.

@frankie567 made a pull request (#6015) that adds the loadData support to the plugin, which fixes the problem with replacing data with another dataset, but not loading data when it was previously defined as an empty array.

Demo

• fixed: https://jsfiddle.net/aninde/1dqceLwj/
• not fixed: https://jsfiddle.net/aninde/5a109gso/

[aninde]

The cases from #3914 will be fixed by a new index mapper in 8.0.0-rc1 (14.09.19)

Demo
Case 1
still fixed in v. 8.0.0-rc1 https://jsfiddle.net/aninde/hLf2nw7v/
v. 7.2.0 fixed: https://jsfiddle.net/aninde/1dqceLwj/

Case 2
still fixed in v. 8.0.0-rc1https://jsfiddle.net/aninde/rgdop7yn/
v. 7.2.0 not fixed: https://jsfiddle.net/aninde/5a109gso/

Data and headers update and renders right after clicking on a button.

[aninde]

Case 2 is still fixed by 8.0.0-beta1 https://jsfiddle.net/aninde/ysmn10xu/