Fix hover handling in QTreeView

Fix hover handling in QTreeView

Cached hoverBranch could get invalid if branches were collapsed or
expanded programmatically, leading to a crash in some situations.

Fixed the logic for updating hovered over branch indicators and
also now update hoverBranch when drawing so that it is guaranteed to be
up to date there - this fixes issues like hover indicator not updating
when the view is programmatically scrolled.

Task-number: QTBUG-27158
Change-Id: I5bd1ad76aee512ad78df33959a84ead16886a47c
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>