core: frontend: allow cancelling new version / extension installs

[ES-Alexander]

Current behaviour

Installing a docker image through the version-chooser or kraken is currently un-cancellable, which can be annoying if you start an installation and realise you don't actually need it immediately (e.g. "I'll do a quick update/install before X - oh, this is taking longer than I expected, I'd rather cancel it for now"). This is particularly annoying for large updates, or slow internet / host computer.

Expected or desired behaviour

There should be an option to cancel an in-progress installation/update (e.g. with a "cancel" button in the "Pulling new version" modal), as there would be with a keyboard interrupt if you were installing a docker image using the terminal.

Prerequisites

• [X] I have checked to make sure that a similar request has not already been filed or fixed.