In the old code a combination of removeRows()/insertRows() was used
to signal a model reset.
Replace this by a single, modelReset signal. This saves a call to
plotPictures() and will allow us to be smarter in the future,
when removing pictures.
Reset-model and remove-items are semantically different.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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