Added various buttons to the panel markdown editor.

Added many new buttons to the markdown editor (h4, h5, h6, youtube, vimeo, image, hr, ul, ol, blockquote, code). 

Not 100% perfect but I guess it's a good start for upgrades until the new version of Kirby panel comes out with maybe more advanced stuff?

Please note that only the FR and EN language files have been modified accordingly.