Use KFileItem::recursiveSize to Display recursize size as total size

Summary:
This allows to display the trash:/ occupied space

CCBUG: 413091

Test Plan: {F8231067}

Reviewers: #dolphin, ngraham, elvisangelaccio, dfaure

Reviewed By: #dolphin, ngraham, elvisangelaccio, dfaure

Subscribers: #baloo

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D28738