Don't updateCell for a new thumbnail

Randall Ruderequested to merge
rsquared/kphotoalbum:B_505015_tc into master

CCBUG: 505015

Here's another scenario that logs "dataChanged() called with an invalid index range":

  • start a fresh demo
  • open the thumbnail view
  • add at least one new image in the demo folder
  • rescan for new images - the message is logged once per new image

This patch only triggers dataChanged() when a thumbnail is updated, not when a new thumbnail is inserted into the cache.

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