Better state management for all webviews

[peterbom]

We recently added the retainContextWhenHidden property to all our webviews.

First, this has the documented drawback that it "has high memory overhead and should only be used when other persistence techniques will not work". We should make an informed decision whether to change this, preferably based on measured memory usage.

Second, you apparently "cannot send messages to a hidden webview". This could be a problem for some of our webviews that perform background operations in VS Code and send updates to the UI (e.g. Periscope, ASO installation, and Create Cluster). We need to test what the current behaviour is, and see whether it needs fixing. If it does, we might be able to build some caching/buffering of messages into our state management approach.