Use classes with fixed fields instead of maps

[michaelspiss]

Is your feature request related to a problem? Please describe.
Currently, a map is used in many places to collect information, for example in ImapFolder._processBody(). This is a poor way to implement things as it requires the developer to remember each value's key.

Describe the solution you'd like
To help with autocompletion and error spotting, classes with fixed fields for each value should be used instead.