feat(panes): introduce options to dim inactive panes and highlight active pane

[aexvir]

Closes #12529 Closes #8639

Release Notes:

• Added option to dim inactive panes
• Added option to highlight active pane with a border

note: don't know much rust, so I wouldn't be surprised if stuff can be done much better, happy to update things after the review. Also, wasn't sure about introducing the new object in the settings, but it felt better than adding two more keys to the root, let me know what you think and if there's a better way to do this. Also happy to get feedback on the text itself, as I didn't spend much thinking how to document this.

[SomeoneToIgnore]

Great, thank you, that's all from me. Seems that the old settings value is somewhere in the Rust struct still? We need to remove that and presumably we're good to go.

[aexvir]

Great, thank you, that's all from me. Seems that the old settings value is somewhere in the rust struct still? We need to remove that and presumably we're good to go.

oh, yep, done; thanks for the review! 🙂

[lougreenwood]

Yes, yes, yes! 🎉

[0x2CA]

Vertical split don't seem very friendly

[cwooldridge1]

how can i set the active border color? I tried

"experimental.theme_overrides": {
    "panel.focused_border": "#ff0000",
    "pane.focused_border": "#ff0000"
}

and it seemed to have no effect

[dumski]

For someone who wonders how to change the active pane color (in settings.json file):

"experimental.theme_overrides": {
  // ...
  "border.selected": "#f00",
  // ...
}

It took me a while to find it in a source code ;/ Cheers