Use forward slashes in path names in mj-map:// urls

The "Copy as Link" context menu command creates a mj-map:// link where local pathnames use the normal Windows backslash character (\).

Unfortunately some software that can recognise URLs does not consider the backslash character to be a valid character, and does not correctly parse the link.

It would be better if forward slashes were output in mj-map:// urls, then they could be handled in other software too. MindManager itself does not seem to mind and will accept forward slashes in local file paths.