Fix corkView background image on Windows

Windows path to the image has '\' path separator instead of '/' which makes
the stylesheet fail. Background images don't appear and console gets spammed with :
Could not parse stylesheet of object corkView(0x27248eb6900, name = "corkView")

manuskript/ui/views/corkView.py

@@ -36,7 +36,7 @@ class corkView(QListView, dndView, outlineBasics):
             background-attachment: fixed;
             }}""".format(
                 color=settings.corkBackground["color"],
-                url=img
+                url=img.replace("\\", "/")
         ))
 
     def dragMoveEvent(self, event):