meld/melddoc.py: Fix windows external open (#354)
When in a Meld folder comparison in Windows, a file or subfolder can be opened externally with default windows application (e.g. explorer for folders, previously selected application for files, or the windows file open option popup for unspecified types).
Right-mouse button click and "Open externally" should produce this behaviour, however currently folders with spaces in the path produce unexpected behaviour such as command line windows opening.
Changing the relevant system call from subprocess.Popen() to the windows-specific os.startfile() fixes this behaviour and handles paths with spaces in them in the expected way.