Ability to edit attachments during composition of a new mail

[romanofski]

Is your feature request related to a problem? Please describe.
When composing a new e-mail, we want to be able to open or edit the attachments. Fixing #182 added a base level feature for handling attachments of parsed mails. In a discussion on #318 we figured that fundamentally attachments could be nested (like you know, MIME mails in general) and showing a tree view of attachments for editing and opening would be more desirable. Biggest benefit would be to avoid any ambiguity which specific part of the attachment should be edited.

Describe the solution you'd like
Don't pick this up if Brick doesn't have a tree widget yet.
Use the tree widget so input for editing/opening attachments happen on the specific Part of the MIMEMessage.

Describe alternatives you've considered
See the discussion on #318

Additional context
None