Support for MS Edge

[Reinmar]

See #423.

[Reinmar]

I'll focus here on the most visible issues and tests which are easy to fix.

2 most important issues are usage of new Range() and non-yet-iterable DOM natives.

[Reinmar]

This is the status after my first batch of fixes:

Edge 14.14393.0 (Windows 10 0.0.0): Executed 3282 of 3282 (11 FAILED) (19.242 secs / 12.634 secs)

This is pretty good. The most worrying tests are those from the model because this is pure JS stuff ;|. I'll debug these. The rest are selection rendering and observer things which also fail in Firefox and could be extracted to a separate ticket.

[Reinmar]

Only 3 red tests remained now.

1. In the renderer – should not be critical. May be just a test error.
2. In the editing controller – unfortunately, when running these tests through a normal procedure selectionChange isn't fired. When running in debug mode, it is.
3. In selection observer – the inf loop catcher doesn't work – may be that in this case Edge also doesn't fire selectionChange properly.

All these tests can be ignored for now. I'll make a PR with the current status of things because it will close ckeditor/ckeditor5-engine#925 immediately. I'll then do more manual tests and focus on issues which will pop out for them.