Add support for title slides

Originally, themes supported so-called title slides. The idea is to add an intro video that contains metadata. This is particularly useful for download formats where there is no player that can show metadata.

Unfortunately, the implementation of title slides was never completely finished. The good news is that an almost finished implementation is still part of the codebase and having a look at it revealed that it can actually be brought alive without too much effort.

To achieve this task, the following work needs to be done:

• Resurrect Configuration->Themes->"Theme Details"->"Title Slide" form in Admin UI
• Extend WOH theme with some control variables needed to implement the workflow doing that actual processing
• Make the workflows respect the settings provided by WOH theme to actually extract a title slide image, add metadata to it, create a video from it and concat it with the video