Improve By Desktop sorting for task groups

Summary:
TaskGroupingProxyModel used to return QVariant() as the
AbstractTasksModel::VirtualDesktop role for task groups,
resulting in TasksModel to sort task groups indiscrimi-
nately at the front of the list. In this patch, it instead
returns the lowest virtual desktop found among the children.

BUG:384747

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11257