Support for multiple open files via split panes

[mkhalila]

Hey, would be cool to be able to have multiple files open. Currently the split editor feature only supports splitting the currently open file into split panes. It would be useful to have support for multiple files so you can see them side by side.

[1024jp]

@mkhalila macOS document-based apps are fundamentally based on a one-to-one relationship between a document and a window, so CotEditor doesn’t plan to support opening separate documents in a split view. For folder documents, the folder and the window form that pair.