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.

BUG:415891