Parse messages from the systemd catalog

Use catalogs to explain messages. To implement this feature, I use
strtok function to parse the message named result->catalog, which exists
in the project.

For instance, I use a whitespace to get the field labels and ignore
them. Then I use "\n" to get the detailed message for the current field.

I also use dim-label style class for these field labels.

https://bugzilla.gnome.org/show_bug.cgi?id=709153