reintroduce All Accounts view and set as default

As an intermediate step to a unified inbox, we should get rid of the account switcher. This makes navigating mail much faster when you have multiple accounts.

Left to do:
* [ ] put in a chooser into the »New message« dialog to pick which email account the message should be sent from. @DeepDiver1975 
* [x] finish #233 (possibility to delete account) by @Gomez – so there is an »Add account« function in the settings
* [x] remove the account switcher dropdown

In the future we should also by default collapse the sets of folders for each account so that it doesn’t take over the whole sidebar.