Move ROOT settings files to config/root directory

[ferdymercury]

Explain what you would like to see improved

ROOT is configured to look for a bunch of configuration files in your "Home" directory, e.g. hidden files .rootrc, rootlogon.C, root aliases, etc. This is the traditional way things were programmed, but now many programs (KiCad, Qt, ...) have been moving towards using the more consensus "configuration" folders, e.g. .config/root in order not to pollute your HOME directory with tenths of files, one for each program you use.

Optional: share how it could be improved

For ROOT7, move things gradually to use "configuration" folders. I understand that this is though quite low priority :)

Some solutions out there that take care of the multiple naming of config folders depending on your OS:

• https://github.com/Malvineous/cfgpath
• https://doc.qt.io/qt-5/qsettings.html#platform-specific-notes
• https://docs.kicad.org/doxygen/classSETTINGS__MANAGER.html#a5e93287a5efd1e9517c1ddcd2f23bb9a

Setup

   ------------------------------------------------------------------
  | Welcome to ROOT 6.27/01                        https://root.cern |
  | (c) 1995-2021, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Jan 12 2022, 10:17:19                 |
  | From heads/master@v6-25-01-2870-gdac9b6398d                      |
  | With c++ (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0                     |
  | Try '.help', '.demo', '.license', '.credits', '.quit'/'.q'       |
   ------------------------------------------------------------------

[Axel-Naumann]

Independently but somewhat related, we may want to store past profiles for branch accesses etc; that's best done inside a dedicated config folder as you suggest here. I.e.: 👍