rhbz#1079672: FileDialogHelper: don't hand out stale preview Graphic
rhbz#1079672: FileDialogHelper: don't hand out stale preview Graphic

The maGraphic is updated from a timer, and if you double-click on a file
then the dialog may exit with the previous file still previewed in
maGraphic, so the wrong image is returned by the dialog.

Change-Id: I99094d85d8d68d5c8a842f52e7039dbbbf095995

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core