WSL support: paths with spaces and URLs

The current WSL support in xdg-open, which launches powershell.exe start, has at least two issues:

• It fails to handle filenames with spaces.
• It doesn't handle URLs.

The attached wsl.patch solves both issues by using explorer.exe rather than powershell.exe start.

I don't seem to have the right to create a merge request and I couldn't find a contributor guide, so I hope that attaching a patch like this is appropriate.