Send new "updateGutter" signal on gutter resize

Some consumers need to know when the gutter width changes.  For
example, the MediaWiki Visual Editor maintains a transparent overlay
that must always line up with the CodeMirror text area.  The gutters
can be resized, for example when line numbering is toggled and
content grows beyond 1,000 lines.  This shifts the text area, and
some styles may need to recalculate based on the new value of
`display.gutters.offsetWidth`.

src/display/update_display.js

@@ -5,6 +5,7 @@ import { displayHeight, displayWidth, getDimensions, paddingVert, scrollGap } fr
 import { mac, webkit } from "../util/browser.js"
 import { activeElt, removeChildren, contains } from "../util/dom.js"
 import { hasHandler, signal } from "../util/event.js"
+import { signalLater } from "../util/operation_group.js"
 import { indexOf } from "../util/misc.js"
 
 import { buildLineElement, updateLineForChanges } from "./update_line.js"
@@ -254,6 +255,8 @@ function patchDisplay(cm, updateNumbersFrom, dims) {
 export function updateGutterSpace(display) {
   let width = display.gutters.offsetWidth
   display.sizer.style.marginLeft = width + "px"
+  // Send an event to consumers responding to changes in gutter width.
+  signalLater(display, "gutterChanged", display)
 }
 
 export function setDocumentHeight(cm, measure) {