calendar-row: Rename calendar-combo-row-item to calendar-row

Currently, the quick add popover implements its own row
widget for each calendar, which duplicates tons of unnecessary
code. This commit streamlines the calendar-combo-row-item row
widget to make it reusable elsewhere.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/362>

src/gui/gcal-quick-add-popover.blp

@@ -9,7 +9,7 @@ template $GcalQuickAddPopover: Popover {
     "quick-add-popover",
   ]
 
-  Box {
+  child: Box {
     orientation: vertical;
     margin-top: 12;
     margin-bottom: 12;
@@ -103,5 +103,5 @@ template $GcalQuickAddPopover: Popover {
         ]
       }
     }
-  }
+  };
 }