Adding new enum to detect the type of a mailbox

[TornaxO7]

This should fix #172

[TornaxO7]

I'm a little bit unsure, if MboxType suits here. Probably a MboxAttribute enum might be better but this would mean that we have to map the enum values of NameAttribute as well, at least, if we'd map the NameAttribute to a MboxAttribue enum, then we could always access the mbox easily with our new-type variant. What do you think about that @soywod ?

[soywod]

I'm afraid you started to develop this from master, so you are not aware about new stuff in development. I changed the target branch and you seem to have conflicts. I refactored well the Mbox (with comments + tests), maybe this refactor will help you in what you wanted to achieve there? It sounds great to me what you did anyway, thanks to take this point it's a tough one

[soywod]

Oh wait, this means we will have one extra call at every command 😕. The user does not have to configure anything at the cost of a bigger response delay. But what if a user defines a mailbox named "MySent" with attributes "Custom(MySent)" as his sending folder? This case would not work. Maybe we should let users configure their mailboxes name. We could set up defaults in case they are not set up (as it works right now on master), it should match 80% of users.

[soywod]

Sth like folder-archive = "Archive", folder-drafts = "Drafts", folder-junk = "Junk", folder-sent = "Sent" and folder-trash = "Trash" in the configuration (global and specific by accounts).

[TornaxO7]

I'm afraid you started to develop this from master, so you are not aware about new stuff in development.

Ooof... yes, you're right. I forgot to do that... I'll close this PR and restart it again.

Sth like folder-archive = "Archive", folder-drafts = "Drafts", folder-junk = "Junk", folder-sent = "Sent" and folder-trash = "Trash" in the configuration (global and specific by accounts).

Yeah, sounds good to me

[soywod]

I switched the default branch to development so that future PRs will point automatically to development instead of master.

[TornaxO7]

Since it seems like that the user has to define in their config which mailbox should be used for what, I'm going to close this PR.