wClipboard/posix: add local files to file list

wClipboard/posix: add local files to file list

Finally we can add a file to the file list once we have got its local
file name decoded. The interesting part here is what we use for the
remote name.

Suppose the user has selected two files in different directories. In
this case we end up receiving a text/uri-list like this:

  file:///home/bob/foo/a
  file:///home/bob/bar/b

We'd expect to see "a" and "b" pasted into the remote session, so that's
what we should use for the remote names: the base names of the files.
These are the parts from the end up to the last directory delimiter.

One tricky point here is that Windows expects the file names to be
encoded in Unicode, but POSIX does not specify any particular encoding
for file names. Operating systems and file systems generally handle the
file names as mostly opaque bytes strings and do not really care what
encoding is used there. There is no portable API to get the encoding,
it's entirely up to the users and the software they use to correctly
interpret the file names. But we need to do something here.

As of 2017, the most widely used encoding for file names is UTF-8. While
there are marginal communities which stick to codepages for legacy
reasons, we can safely assume that most of the time the file names will
be encoded in UTF-8. In fact, popular desktop environments like GNOME
also assume this. So that's what we will do here as well.