PPTX writer: support notes field in metadata for title slide (#11396)

This adds support for a `notes` field in the YAML metadata block
that will be used as speaker notes for the title slide in PowerPoint
output.

Previously, there was no way to add speaker notes to the title slide
since it is generated from metadata rather than from content blocks.
The `::: notes` syntax only works for content slides.

Example usage:

    ---
    title: My Presentation
    notes: |
      Welcome everyone to this presentation.
      Remember to introduce yourself.
    ---

Closes #5844 (for PPTX output).

Co-authored-by: Chris Callison-Burch <[email protected]>

1 files changed, 7 insertions, 1 deletions

diff --git a/src/Text/Pandoc/Writers/Powerpoint/Presentation.hs b/src/Text/Pandoc/Writers/Powerpoint/Presentation.hs
index 9e7bcb04f..b969c645b 100644
--- a/src/Text/Pandoc/Writers/Powerpoint/Presentation.hs
+++ b/src/Text/Pandoc/Writers/Powerpoint/Presentation.hs
@@ -887,6 +887,12 @@ getMetaSlide  = do
   subtitle <- inlinesToParElems $ lookupMetaInlines "subtitle" meta
   authors <- mapM inlinesToParElems $ docAuthors meta
   date <- inlinesToParElems $ docDate meta
+  -- Get speaker notes from metadata "notes" field
+  let notesBlocks = lookupMetaBlocks "notes" meta
+  speakerNotes <- if null notesBlocks
+                  then return mempty
+                  else local (\env -> env{envInSpeakerNotes=True}) $
+                       SpeakerNotes . mconcat <$> mapM blockToParagraphs notesBlocks
   if null title && null subtitle && null authors && null date
     then return Nothing
     else return $
@@ -894,7 +900,7 @@ getMetaSlide  = do
          Slide
          metadataSlideId
          (MetadataSlide title subtitle authors date)
-         mempty
+         speakerNotes
          Nothing
 
 addSpeakerNotesToMetaSlide :: Slide -> [Block] -> Pres (Slide, [Block])