Drop selection if we hide the layer

If we move the selection (or just click it) and then hide the layer, we should drop the selection, because the MovingPixelsState is still running and if we change layer we'll cut the original selection (which is confusing because we didn't cut the selection, just hide the layer).

Reported here: https://community.aseprite.org/t/copy-paste-deletes-areas-in-hidden-layer/20621