BUG:371592 fix import image sequence dialog

BUG:371592 fix import image sequence dialog

I noticed this bug came from mainWindow->showOpenFileDialog();
I first replaced it with a standard QFileDialog::getOpenFileName();, which worked, but didn't have all the properties set.
So I copied showOpenFileDialog there, and issue is gone.
It's maybe not the cleanest fix, but it does work.