Allow opening scenes in text editor

[caryoscelus]

It is sometimes useful to read the scene code or even edit it. But looks like now it's only possible through external means.

To avoid confusion inside editor, it'd be nice to have separation in document list (e.g. groups like "scripts", "scenes", "misc")

[bojidar-bg]

Is this (a slightly far-fetched) duplicate of #1390, per chance?

[caryoscelus]

I would say it's more of an addition to it, because it means adding some special action of opening scene in text editor (rather than just using file->open) and hopefully some syntax highlighting.

[akien-mga]

For now I'll close this as strictly dependent on #1390 (can't happen without the former being implemented), and I'll add a mention there of potential scene-specific features in the text editor. Eventually if/when #1390 is being worked on, this issue could be reopened if need be to track the scene-specific changes separately.