Implement asynchronous saving of Krita documents
Now all the saving operations work in the following way: 1) Make a shallow copy of the image 2) Create a separate document with it 3) Save it in a separate thread in background The functionality basically works, except the following parts: 1) Cloning the image "forgets" some image stuff. I don't have a list of what is forgotten yet (layer styles?). 2) Closing the image while Krita is saving will most probably cause a crash. 3) D&D of layers is not tested, though it should work. 4) There was some option to provide "real name" for json export plugin, it is not supported anymore.
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