Despite the existence of a logo, Manuskript lacks an icon file that can
be used on Windows operating systems. This new icon file was created
based on the existing PNG files for 16px, 32px, 64px, 128px and 256px.
Add "Help -> About" menu entry that invokes an "About" dialog window
for Manuskript.
Show details such as logo, version, website, copyright, license, and
software versions in use (Python, PyQT, and Qt).