Show progress while resolving next level hover

[mjbvz]

Testing #210904

When a hover provider takes a few seconds to resolve the next level hover, there is not currently any feedback that next hover is being resolved. I suggest we show a spinner and also prevent the user from clicking the +/- buttons in these cases

[aiday-mar]

The hover verbosity logic has been changed so that it is now possible to make consecutive requests while a request is still pending in #213317. This will end the current request and start a new request. With this logic, the + and - buttons need to be enabled.

We could still consider adding a loading spinner however to show that work is done. I am not sure however where to place it. Indeed there is little space in the hover. The hover can be of variable size - as tall as the height of 1 line for example (19 pixels only), so there isn't vertical space where to place the spinner. We could place the spinner to the left or to the right of the +/- icons but that could look a bit strange, as the hover expands horizontally to acomodate a spinner and then contracts when loading is finished.