Persistence mechanism

[JonathanTurnock]

The application needs an improved persistence layer, but it still needs to maintain support for multiple tabs having different content.

It should not increase the friction but it's quite unmanageable having lots of tabs and loosing progress as it is.

Support should initially be for 'Device' (Local Storage) but should also support future scope for adding GitHub integration etc.

Initially we can probably roll with the concept of folders&files where an unsaved file is a scratch but when saved it opens a file explorer on the left and uses can flip between files.

GitHub persistence is probably an extended action to propagate changes in local storage to the GitHub api.