Add subtitle synchronization on HTML video player

[redSpoutnik]

This PR add the ability to manually set an offset to some subtitles in HTML video player:

• subtitle stream codec must be compatible with vtt (e.g. srt) or ass.
• subtitle stream display mode must be external (subtitle extraction must be allowed).

Changes

• If previous conditions are fullfilled, a slider shown between subtitle and parameters buttons.
• Current offset range is -30 sec to +30 sec.
• On selected subtitle changes, the offset is reset to 0.
• This feature works only in UI and doesn't require backend call.

Tested on Linux Mint 18.2 with :

• Brave Browser 0.61.51
• Google Chrome 73.0.3683.75
• Firefox Quantum 65.0.1

Issues

May partially solve jellyfin/jellyfin#130.

[redSpoutnik]

[cvium]

I'd prefer if it wasn't a slider and it should be inside the options menu tbh. The current placement is a bit crowded

[JustAMan]

I would say it should be inside CC menu which appears when one clicks respective button.
Also having a field which could be filled manually (in addition to a slider) would be good.

[redSpoutnik]

Thanks, I will take the time to test those changes and I will give you some feedback.

[dkanada]

One quick note, I fixed the CI issue here in another PR so you won't have any issues in the future :) The backend changes look good, I agree about moving it out of the main bar though. I would say inside the settings button since that one is for general options. I think the subtitle and audio buttons should stay as track selection.

[redSpoutnik]

Hi all,
I want to share with you my recent tests on this PR.

To avoid grouping slider and editable field in a tight menu item, I took inspiration from kodi subtitle offset.

If a subtitle track is selected, a localized "SubtitleOffset" option appears in the settings:

This option display a panel with the slider and a text field:

The slider has the exact same behavior as before:

The textfield is editable:

• Slider and textfield values are synchronized.
• On mouse inactivity the panel fades with bottom controls, except if the textfield is being edited.
• If the panel is closed using the cross, it will not show up until the "SubtitleOffset" option is clicked again, but the subtitle offset remain.

What do you think?

[dkanada]

@redSpoutnik there seem to be a few conflicts but I think this is ready to merge once those are fixed up.

cc @JustAMan @cvium for comments

[anthonylavado]

Ready to put this in as soon as the conflicts are sorted.

[redSpoutnik]

@dkanada @anthonylavado Conflicts merged :) but be aware that I let the "subtitle offset" button in settings, whereas the "subtitle settings" has been removed.
This PR doesn't include translations of the button's name btw.

[anthonylavado]

This PR doesn't include translations of the button's name btw.

We'll want to make an issue to track this!
Thanks for the great PR @redSpoutnik 👏