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.
this is so wrong - will work on this ....