add an Observable version of boundingbox

[aplavin]

Feature description

To draw background below a text label, one can do

t = text!(...)
translate!(t, 0,0,10)
poly!(boundingbox(t, :pixel), space=:pixel, ...)

but it only works for static plots. When anything changes in the plot, the boundingbox remains the same – it's a plain value, not an Observable.
It's also non-trivial to explicitly list the observables boundingbox depends on, they are both axis properties and the text properties.

Would be useful to have an observable version :)
I'd be happy to open a PR adding it, if someone can tell what exactly it should depend on...