build.py

Remove the GNOME dependencies and have the --qt argument always
require a value when used (exclusively to specify a Qt installation
prefix).

* build.py (ZYPPER_CLIENT_GNOME_DEPENDENCIES): Delete variable.
(ZYPPER_CLIENT_QT_DEPENDENCIES): Rename to...
(ZYPPER_CLIENT_DEPENDENCIES): ... this.
(DNF_CLIENT_GNOME_DEPENDENCIES): Delete variable.
(DNF_CLIENT_QT_DEPENDENCIES): Rename to...
(DNF_CLIENT_DEPENDENCIES): ... this.
(APT_DEPENDENCIES): Delete 'gnome-icon-theme-symbolic'.
(APT_CLIENT_GNOME_DEPENDENCIES): Delete variable.
(APT_CLIENT_QT_DEPENDENCIES): Rename to...
(APT_CLIENT_DEPENDENCIES): ... this.
(PACMAN_CLIENT_GNOME_DEPENDENCIES): Delete variable.
(PACMAN_CLIENT_QT_DEPENDENCIES): Rename to...
(PACMAN_CLIENT_DEPENDENCIES): ... this.
(run_dependencies): Remove usage of args.gnome.  Streamline use of
--qt; now it's only used to specify a custom Qt prefix.
(run_uninstall): Get rid of CLIENT_SUFFIX, adjusting users
accordingly.
(run_install): Delete '-c' option of install script.
(run_run): Delete hacks used to build the now-deprecated client-macos
on macOS.  Get rid of client_suffix.  Adjust client path to run.
(run_stop): Get rid of suffix.
(execute_script): Test falsy values as intended.
(parse_args): Delete '--gnome' argument.  Adjust doc of '--qt'.
(main): Get rid of 'parsed_args.gnome'.
* guix/manifest.scm: Streamline and update dependencies.
* scripts/install.sh: Remove '-c' option.  Remove conditional branches
handling GNOME client.
* guix/channels.scm: New file.

Change-Id: I26898c3331467bfc4ab63d3e9df1d50560c287c2