Optimize chart-shape and made loading charts double as fast.

We compress multiple proxyModelStructureChanged() emitted during loading charts now into one single call at the end of the loading process what gives us a huge speed-up when loading charts.
With my test-document ( Harmattan-Columbus_SW_Maturity_Applications_Release-W19_10.xls ) loading all the charts in that Excel document is done more then double as fast now (before ~40-50ms per chart and after this patch ~10-20ms).