Use native separators where appropriate

I guess this is it. Closes #305

After this is applied, you will need to cast to native separators in 2 situations:

1. When using libtorrent API directly
1. When showing paths to user

Also forget about `QDir::separator()` - use normal slash

All classes work with qt-style separators internally (with some exceptions like WinAPI calls) and only use native ones in two cases above, also all 'mission critical' classes will accept both native and qt-style separators: session, handle, preferences, fsutils.
Support for smb in Linux look impossible to implement using `QFileDialog`.

Will probably need squashing.