If we delete some tabs, and then add a new one, all of the previous tabs are restored. Similarly - if a user re-arranges their tabs they get put back into a single stack when a new one is added.

This is because our tabs state isn't in sync with the live set.

We should overcome this in one of two ways:

• don't have our own set of tabs that get transformed into the Model. Start with an empty model at the start, and add new tabs using doAction events
• track tabs in the UI being closed, and remove the relevant entry in the `tabs object. Note: we'll also have to track if/when the layout gets modified by the user, and cacheing the new layout. Then, when adding a new tab - add it to the most recent tab-set.

I think the first one is more robust. It will also give us the opportunity (in the future) to persist the tab arrangement for when the app next opens.