add support for <subtitle> tag in FictionBook

Currently, Okular silently ignores <subtitle> tags in FictionBook files. This PR adds support for subtitles so they are rendered like <title>, but in a smaller font.

Attention: I was unable to test these changes. Neither Okular repository nor website contain any information on how to build and run Okular from source. I figured out that it can be built via cmake --build . from repository root. But when I run ./bin/okular, it loads plugins from host system instead of loading them from <repo>/bin/plugins/okular. Created #41 (closed) for this lack of documentation.