libtizplatform: use $XDG_CONFIG_HOME and $XDG_CONFIG_DIRS to locate tizonia.conf

[juanrubio]

From https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

There is a single base directory relative to which user-specific configuration files should be written. 
This directory is defined by the environment variable $XDG_CONFIG_HOME.

There is a set of preference ordered base directories relative to which configuration files should be searched. 
This set of directories is defined by the environment variable $XDG_CONFIG_DIRS.

$XDG_CONFIG_HOME defines the base directory relative to which user specific configuration files should be stored. 
If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used.

$XDG_CONFIG_DIRS defines the preference-ordered set of base directories to search for configuration files in addition to the $XDG_CONFIG_HOME base directory. 
The directories in $XDG_CONFIG_DIRS should be seperated with a colon ':'.

If $XDG_CONFIG_DIRS is either not set or empty, a value equal to /etc/xdg should be used.