update_versions remodel

The current app version model has become a complex kludge. This is a proposal for a new design.

Directory structure:

apps/
  appname1/
    1.0/
      windows_intelx86/
        (files)
      windows_intelx86__cuda/
        (files)
      i686-apple-darwin/
        (files)
      ... other platforms
    1.1/
    ... other versions
  appname2/
  ... other apps

The version directories have names of the form PLATFORM or PLATFORM_PLANCLASS. The contents of a particular directory are:

• application files
• signature files (FILENAME.sig)
• a "version description file", version.xml

The structure of version.xml is:

<version>
  <file>
    <physical_name>PNAME</physical_name>
    [ <logical_name>LNAME</logical_name> ]
    [ <main_program/> ]
    [ <copy_file/> ]
  </file>
  ...
</version>