State in extension popup

[bonomat]

Since the extension popup is only short living, is completely stateless atm. State is only stored in the background script.

This can be an issue as seen in #226 : The loan signing process is a 3 step process:

1. sign loan
2. download backup file
3. submit to bobtimus

If the user loses focus of the popup, e.g. because he wants to check if the backup file was successfully downloaded, the state is gone. When he re-opens the popup the signin process starts from the beginning.