Add possibility to change Blackbar detector mode with JSON API

[ld892012]

Feature request

What problem does this feature solve?

Some modes works better for some sources (OSD for TV, Default for old games, LetterBox for videos,...)

What does the proposed API look like?

Something like "command : blackborder , mode : default/classic/osd/letterbox"

How should this be implemented in your opinion?

Like you want, when you want, if you want !

Are you willing to work on this yourself?

If my knowledge would have allowed to me