project: Add `DESCRIPTION` parameter

It is quite often the project description has used in a real world software.
Examples include:

* part of a help screen of the application
* builtin resources (`*.rc` files, data for "About" dialog of a GUI app, & etc)
* most generators for CPack can use it
* it could be used by documentary software (Doxygen, Sphinx) which is usually
  integrated to CMake based projects via `add_custom_target()`

Now `project()` call learned an optional `DESCRIPTION` parameter with a
short string describing a project. Being specified, it would set the
`PROJECT_DESCRIPTION` variable which could be used in `configure_file()`
or whatever user wants.  Also `PROJECT_DESCRIPTION` is a default value
for `CPACK_PACKAGE_DESCRIPTION_SUMMARY`.

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