Expand the "pinned row" concept to use a predicate instead of a property

[cristiano-belloni]

Description

Allow predicates instead of pinned property.

Motivation and Context

In order to make the grid more configurable, we expanded the concept of "pinned row" to use a predicate that says if the row is always shown, even when the parent is collapsed or not. This is built on top of #18 and it will behave the same (i.e. it will support one level of depth)

How Was This Tested?

Manually, see this updated gist.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

  Checklist:

• [x] My change follows the style of this project
• [x] My change requires a change to the documentation
• [x] I have updated the documentation accordingly
• [x] I have read the contribution guidelines
• [ ] I have added tests to cover my changes
• [ ] All new and existing tests passed