sysutils/py-ranger: Respect hier(7) and make concurrent safe

Documentation related files were installed into ${PREFIX}/doc instead of
${PREFIX}/share/doc. [1] Take this opportunity too to make the port
concurrent safe as it installs beside documentation also scripts and a
desktop entry.

Introduce an appropriate workaround for the desktop entry by prefixing the
filename after the corresponding Python flavor and also take care for the
script executable. Add "desktop-file-utils" to USES to update the desktop
database and reduce warnings in the stage QA.

Remove "python_OLD_CMD" that is no longer required and place a "do-test"
target with the required dependencies to easen QA related tasks in future.

PR: 237775
Reported by: Ting-Wei Lan <lantw44@gmail.com> [1]
Approved by: maintainer timeout (14 days)