Support "profiles" for multiple configurations

This adds support for reading configuration files from an alternate
configuration directory. For that, argv[0] is inspected, and if it
is different from "dwt", a different D-Bus application ID is used
(org.perezdecastro.org.${argv[0]}) and the configuration directory
is changed to be $XDG_CONFIG_DIR/dwt.${argv[0]}. This allows to have
different sets of settings, and let dwt pick one according to the
name it has been invoked as.
Allow setting the default command with a setting

This is useful to instruct dwt to use by default a command other than the
user's login shell. It is particularly useful with multiple profiles, e.g.
to launch applications that need a terminal by their name, with their own
fonts and settings. The setting is called "command".