add legacy version number to "About Pd"

When a user clicks "Help->About Pd" the file in doc/about.pd pops up and shows the version and other useful info.

However, it doesn't currently show the legacy version number from Pd vanilla. That version can be informative for users who want to figure out compability with Pd vanilla.

So maybe that version number can have a comment heading saying something like, "Roughly compatible with Pd Vanilla Version x.x.x" where "x.x.x" is queried using [version(--[pdinfo].