Fix partial TermGroup decoration

[chabou]

First TermGroup of each window is decorated here

But when a TermGroup add a sub-TermGroup in a SplitPane, it is not decorated.

This PR fix this.

[chabou]

Warning!
This PR could break plugins which assume that decorateTermGroup and getTermGroupProps are called only for each RootTermGroup.
It could be better to not merge this PR and warn somewhere in docs than only main TermGroup is decorated. I think that @ppot rework will fix this partial decoration.

[chabou]

Maybe hyper-pane is the only plugin which assumes that 🤣
In this case, I can fix it and we could merge this PR

[rauchg]

@chabou, this is the behavior I'd expect. So let's take our chances!

[chabou]

@rauchg I've scripted a check for all plugins listed in https://github.com/bnb/awesome-hyper

Only 8 plugins use getTermGroupProps. 7 use this hook only to pass down props without any action. So they'll work without problem. The only one which use some logic and will not work is mine: hyper-pane. I'll fix it very soon.

No plugin use decorateTermGroup.

We can conclude that this PR will not break anything 🎉