support title attribute for callouts

Author: jjallaire

src/resources/editor/tools/attrs.yml

@@ -20,6 +20,8 @@
   contexts: [div]
   filter: "\\.callout"
   completions:
+    - value: 'title="$0"'
+      doc: "Title displayed in callout header"
     - value: 'icon="true"'
       doc: Include an icon in the callout.
     - value: 'icon="false"'

src/resources/filters/quarto-pre/callout.lua

@@ -28,7 +28,10 @@ _quarto.ast.add_handler({
   -- and returns the custom node
   parse = function(div)
     preState.hasCallouts = true
-    local title = resolveHeadingCaption(div)
+    local title = markdownToInlines(div.attr.attributes["title"])
+    if not title or #title == 0 then
+      title = resolveHeadingCaption(div)
+    end
     local old_attr = div.attr
     local appearanceRaw = div.attr.attributes["appearance"]
     local icon = div.attr.attributes["icon"]