Fixed escaping exception with thumbnail paths and optional property patterns

[sualfred]

@mikesilvo164

If you have set <propertypattern> and a custom thumbnail it will cause an exception of re.sub() because a path like g:\test.jpg has a backslash included.

This backslash needs to be escaped. This small line fixes the issue and will handle the \ as string.

[MikeSiLVO]

Thanks, will send for update in Kodi repo. I don't actively check for PR's since I'm supposed to get emails and notifications but I am currently not getting either 😞