Various fixes and improvements

## Work done
* Finalized imap folder handler initial version
* Improved the fixtures
* Minor style fix
* Minor store fixes (some data were not immutable)
* Fixed the menu in responsive mode
* `selectedMailbox` should now be always correctly set and handled by the UI

## New question
* since we can't know which imap folder is what (we can't know a folder is an inbox or draft or trash or ... ), we can't put the right icon to the right folder. So we need to consider other options to distinguish them (colors ?)