Refactor the folder expansion system. Main effect: instead of having a list of m_exp...

Refactor the folder expansion system.  Main effect: instead of having a list of m_expandedViews which cannot yet be deleted, we more accurately identify the current source of the drag operation if any and store only this for later deletion.

This also fixes folder-expansion not working in Column view.

svn path=/trunk/KDE/kdebase/apps/; revision=916701