Restore NavigationToolTips for file items in projects manager

Summary:
With the introduction of ProjectModelItemDelegate the tooltip events for
items have been handled by its inherited QItemDelegate::helpEvent(), thus
somehow no longer bubbling to ProjectTreeView::event().
As a result no longer the NavigationToolTips were shown for supported file
items, only the full path as delivered by ProjectModel.

This patch restores that behaviour, by moving the respective code to the
helpEvent handler of ProjectModelItemDelegate.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8410