Fix WTreeView loading. See #11240

[jhnstrk]

When rows are removed, WTreeView::modelRowsAboutToBeRemoved calls isExpandedRecursive to determine if rendered tree nodes are impacted by the removal. However, isExpandedRecursive was always returning false for the root node. This seems odd, when the root node is always expanded. This behaviour caused a bug because the removedHeight_ was not being set. This lead to the childrenHeight not being updated, and the loading isn't triggered.

Switching isExpandedRecursive to true for the root index fixes the issue ... but maybe it was intentionally set like this, but I can't see why.

[emweb]

Thanks. Having it return false on the root index doesn't make any sense to me either.

I manually merged this with some changes (adding your description to the commit message and adding release notes), so I'm closing this PR.

Roel