BUG: Fix "transform file not marked as modified on save".

Call StorableModifiedTime in ProcessMRMLEvents() when the transform matrix is internally modified.
Fixes #3303

From: Yuzheng Zhou <yuzheng.zhou@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22308 3bd1e089-480b-0410-8dfb-8563597acbee