[lint addon] Use separate CSS classes for common lint styles

This changes lint.css to be less reliant on the predefined severities
(error and warning), in turn making it easier to define custom ones.
Now all that needs to be done in order to define a new severity, e.g.
`note`, is to add the following CSS:

```css
/* underline */
.CodeMirror-lint-mark-note {
  background-image: ...;
}

/* icon */
.CodeMirror-lint-marker-note, .CodeMirror-lint-message-note {
  background-image: ...;
}
```

Previously, it was necessary to copy many styles that were only
available under the `CodeMirror-lint-*-error` and
`CodeMirror-lint-*-warning` classes.

addon/lint/lint.css

@@ -25,7 +25,7 @@
   -ms-transition: opacity .4s;
 }
 
-.CodeMirror-lint-mark-error, .CodeMirror-lint-mark-warning {
+.CodeMirror-lint-mark {
   background-position: left bottom;
   background-repeat: repeat-x;
 }
@@ -40,7 +40,7 @@
   background-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAQAAAADCAYAAAC09K7GAAAAAXNSR0IArs4c6QAAAAZiS0dEAP8A/wD/oL2nkwAAAAlwSFlzAAALEwAACxMBAJqcGAAAAAd0SU1FB9sJFhQXEbhTg7YAAAAZdEVYdENvbW1lbnQAQ3JlYXRlZCB3aXRoIEdJTVBXgQ4XAAAAMklEQVQI12NkgIIvJ3QXMjAwdDN+OaEbysDA4MPAwNDNwMCwiOHLCd1zX07o6kBVGQEAKBANtobskNMAAAAASUVORK5CYII=");
 }
 
-.CodeMirror-lint-marker-error, .CodeMirror-lint-marker-warning {
+.CodeMirror-lint-marker {
   background-position: center center;
   background-repeat: no-repeat;
   cursor: pointer;
@@ -51,7 +51,7 @@
   position: relative;
 }
 
-.CodeMirror-lint-message-error, .CodeMirror-lint-message-warning {
+.CodeMirror-lint-message {
   padding-left: 18px;
   background-position: top left;
   background-repeat: no-repeat;

addon/lint/lint.js

@@ -83,10 +83,10 @@
 
   function makeMarker(cm, labels, severity, multiple, tooltips) {
     var marker = document.createElement("div"), inner = marker;
-    marker.className = "CodeMirror-lint-marker-" + severity;
+    marker.className = "CodeMirror-lint-marker CodeMirror-lint-marker-" + severity;
     if (multiple) {
       inner = marker.appendChild(document.createElement("div"));
-      inner.className = "CodeMirror-lint-marker-multiple";
+      inner.className = "CodeMirror-lint-marker CodeMirror-lint-marker-multiple";
     }
 
     if (tooltips != false) CodeMirror.on(inner, "mouseover", function(e) {
@@ -114,7 +114,7 @@
     var severity = ann.severity;
     if (!severity) severity = "error";
     var tip = document.createElement("div");
-    tip.className = "CodeMirror-lint-message-" + severity;
+    tip.className = "CodeMirror-lint-message CodeMirror-lint-message-" + severity;
     if (typeof ann.messageHTML != 'undefined') {
       tip.innerHTML = ann.messageHTML;
     } else {
@@ -183,7 +183,7 @@
         if (state.hasGutter) tipLabel.appendChild(annotationTooltip(ann));
 
         if (ann.to) state.marked.push(cm.markText(ann.from, ann.to, {
-          className: "CodeMirror-lint-mark-" + severity,
+          className: "CodeMirror-lint-mark CodeMirror-lint-mark-" + severity,
           __annotation: ann
         }));
       }