Suggestion: alternative mode for right hand pane

[joningold]

To steal a trick from Inform 7's split-pane IDE: the right pane can be switched from the player view to another code view, so you can look at different parts of the same file at once (and potentially, at two files at one).

[joethephish]

For implementation:

• Requires generalising the usage of playerView.js and editorView.js.
• The layout of the two panes are currently hard coded in index.html and in the CSS. Instead, we could create CSS classes for .leftPane and .rightPane. And I'd suggest at the same time as this task, add .singlePane too, to go full screen in a particular view.

Once the code is more flexible / generalised, we'd add some (xcode-style?) UI to toggle the two panes, which would be managed from controller.js.