gui/event-popover: Apply style to italicize

Using the italic attribute in the label property made the
attribute translatable, which is undesired.

Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1427

src/gui/gcal-event-popover.blp

@@ -95,13 +95,14 @@ template $GcalEventPopover: Popover {
 
     Label placeholder_label {
       vexpand: true;
-      label: _("<i>No event notes</i>");
+      label: _("No event notes");
       margin-top: 12;
       margin-bottom: 12;
       use-markup: true;
 
       styles [
         "dim-label",
+        "no-events",
       ]
     }