Converted os.path to pathlib and all #! lines to python3 (ref #32).

Converted all print statements to functions for Python 3 (ref #32).

• Ensured progress bar is silent when maximum value is zero.

• Added progress bars for all long-running operations (closes #18). ...

• Added new ProgressBar class.
• Switched from subprocess to pexpect for running shell commands.
• Added general shell quoting for debugging output of commands.
• Rewrote the way command strings and argument lists are generated.
• Removed “-loglevel error” from ffmpeg commands to enable capturing progress information (pexpect doesn’t echo the output anyway).