Add methods to expand/collapse multiple levels of items without a performance penalty.

[grokys]

Previously the only way to e.g. expand all items in a hierarchical TreeDataGrid was to expand each item one by one. This was very slow as after each expansion the flattened list of rows needed to be recalculated.

Added the follow methods which speed this up:

• ExpandAll: to expand all rows recursively
• ExpandCollapseRecursive: to expand or collapse all rows recursively based on a predicate
• CollapseAll: to collapse all rows recursively

[alvaro-berruezo-unity3d]

Tested the new methods to expand/collapse a large number of rows (~60k) with a good amount of depth. The methods work as expected and the performance is greatly improved

[neondev9]

Included in recent 11.0.10. Thanks a lot for this very useful methods.