Find configs also from XDG_CONFIG_DIRS

[jluttine]

Great that rofi-file-browser looks for the default config in the standard location under $XDG_USER_CONFIG_DIR. However, this is only the user config directory. It would be nice if rofi-file-browser also respected standard system config directories given in $XDG_CONFIG_DIRS. This way it'd be possible to define system-wide defaults (for instance, in a desktop environment).

I guess it would make most sense if the both the user and system config files are taken into account by merging them so that the user configs override the values in the system config. Not so that the system config file is ignored entirely if a user config file exists. But what if multiple of those system config dirs contain a config file? Should only the first path be used or should they be merged too? I wonder if there's some standard practice for this.