Backport aa254c9e6f2d1ecfa2512111746a77c05ba9717f from libepubgen,
implement XMLRubyContext, XMLRubyTextContext, XMLRubyBaseContext.

Character formats of ruby text, ruby alignment, and ruby position
are not implemented yet.

Change-Id: I6c3708e6bc8e9e36a68171a037fd393f45d8d34f
Reviewed-on: https://gerrit.libreoffice.org/53408Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>