Fix hangup when loaging image with generator/file layers
When loading layers we shouldn't "block" updates, we shoudl just drop them. When loading is finished, KisImage::initialRefreshGraph() will be called anyway, and the projections will become initialized.
To implement proper dropping of the updates I had to (finally) implement nested projection updates filters. Before the patch, there could be only one filter installed. Now they can be stacked in a nested way. What is more the caller now gets a special ticket ("cookie") for the installed filter, so the system can catch interleaved/non-nested filter handling, which is prohibited.