notes: Add placeholder text if notes section empty

Move the textview box into an overlay, with a label as the overlay
child,
that appears only when the textview is empty, and remove the "Notes"
title for the preference group with the notes row in the event dialog.

This better indicates the notes is an input field to the user, compared
to the design before.

Closes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1081
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/714>

src/theme/style.css

@@ -400,6 +400,10 @@ monthpopover > box {
   padding: 8px;
 }
 
+.notes-section label.placeholder {
+  margin: 14px;
+}
+
 .notes-section box > textview > text {
   background: none;
 }