It's unclear why the previous was added (by me) 12 years ago.
Maybe the code in main_handle_filename() or after has been
changed/improved so that it works now or the previous test case for
opening files from the command line was wrong.
This happens only when starting a new instance of Geany from command
line. Opening files in a running instance works fine with and without
this change.

Closes geany#2652.

When passing filenames from the command line on Windows, the filename
is in system locale. If it is a relative path, we make it absolute with
the current working directory which is in UTF-8 encoding.
So we need to convert the current working directory also to system
locale, join the filename and then convert both back to UTF-8.

Closes geany#2652.