Open files as read-only

[dacap]

It would be nice to have an option to load files as read-only, or load then as read-only by default when something strange happens in the loading process, mainly when loading .aseprite files, e.g. some unsupported information/chunk inside the file was discarded, so we don't want to overwrite the content by default.

[martincapello]

What if we just show a warning message as suggested in #3812 and then the loaded file is renamed as <original filename>-incomplete.aseprite for instance, this way the file can still be writable and there is no chance to mistakenly save the file and overwrite its content.

[dacap]

Probably we could rename the file (so it's harder to overwrite the original file) but still load it as read-only (so the user must use File > Save As explicitly or something similar).

Generally I think that saving this file will result in an unexpected result for the user and lost of information, I'm still not sure if we should or not completely disable the save option in this case (the only proper solution to continue for the user is updating Aseprite to the latest version, that is the most important part of the message in #3812, that the latest version of Aseprite must be used to edit the file without losing information).