Update editor/set-line to CodeMirror 4 API

`lt.objs.editor/set-line` used the CodeMirror method `.setLine`, which
is deprecated as of v4.

The updated `set-line` uses `.replaceRange` instead, as suggested in
the CodeMirror v4 upgrade guide.
http://codemirror.net/doc/upgrade_v4.html#upgrade.
Merge pull request #1472 from ndr-qef/cm4

Update editor/set-line to CodeMirror 4 API