Add volume slider to mute button

[camvrgs]

Addresses Issue #160 , adds a vertical volume slider to the mute button, which updates the video volume level using setVolume (available in all handlers of H5P.Video).

[otacke]

@camvrgs Very cool addition! Thanks! I hope the H5P core team will find time to take a look at this pull request soon.

The one thing that they'll probably comment on is accessibility, as you can neither access the slider with the keyboard (you now have ) nor is it announced via readspeakers. You might want to check on YouTube how it's done there, maybe that's an option here as well.

[camvrgs]

@otacke Great suggestion, you're right that would make it more of a complete solution. I'll take another look, thanks!

[Nugatp]

Hey everyone. Is this project inactive? It would be amazing if we could merge this!

[fudgepop01]

Hi! I came looking here after getting fed up with the lack of a slider while watching an assigned video at my university ahahah - what's odd to me is that the capability to adjust the volume is already there. There is clearly a video.setVolume(0 to 100) when I stepped through with the developer tools, but it's just inaccessible.

Technically I can just adjust the volume of my computer but that just seems cumbersome for a feature that is already theoretically available. Have the maintainers taken a look at this?