Studio/place markers: annotate place marker (optional string). Re #155.

Without annotating place marker, Mypy flags track locator call found in place marker script as type mismatch. Therefore flag place marker as an optoinal string.

addon/appModules/splstudio/__init__.py

@@ -1767,7 +1767,7 @@ def _libraryScanAnnouncer(self, count: int, announcementType: str) -> None:
 				ui.message(_("{itemCount} items scanned").format(itemCount=count))
 
 	# Place markers.
-	placeMarker = None
+	placeMarker: Optional[str] = None
 
 	# Is the place marker set on this track?
 	# Track argument is None (only useful for debugging purposes).