If a non-existing file is specified with the command "core:open-file"
a new document is opened with the given filename provided the directory
already exists.
The flag new_file is set to true in the Doc instance.
The file will be actually created only when the "save" command is used.
The document will be marked with the "*" event when no changes are done
to mean that it is a new file and is not yet saved.
The function common.normalize_path now process the .. and . in the
filename. Before was not needed because system.absolute_path already
get rid of them but now we need to have the absolute path of files
that not yet exists so we cannot use system.absolute_path.
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