Major performance improvement when turning previews off in the file dialog.
Review Request #129159 - Created Oct. 12, 2016 and submitted
Added a method for this KDirModel so it can emit dataChanged only once (or once per dir in a tree) rather than for every item. CCBUG: 346403
kdialog --getopenfilename /dir/with/many/images
Clicking on the file dialog's "preview" button to turn them off.
Before: 100% CPU for a minute or so
After: immediate response