feat: Support AddSubtitle json-rpc API

[pespin]

This new button allows passing a URL to Kodi to set up subtitles from
it.

See xbmc/xbmc#18445

TODO:

• Add a new svg file for it (where did you get them from?)
• Proper french transltion strings (would be great if you could provide them when reviewing)

[regseb]

Thank you for the interesting proposal. But do not add this sending at the same level as cast a video / music. It's ambiguous to have three options for videos / music and one for subtitles. You should add a button in the popup that opens a dialog box (as for Send text).

For icons, I get it from Photon's collection. If it doesn't exist, I create it with these guidelines. Here is an icon for the subtitle: subtitle.zip

In keyboard controls of Kodi, the keyboard shortcut S is associated with the shutdown menu. You should use the letter T.