list layout for inbox
Provided menu item for refreshing folders
Can show email detail view on list double click
Highlight unread emails
Reset detail view before loading message

This avoids, that the user sees the previously shown message while the
new one gets loaded.
Updated View constraints
Mark message as read before show in detail view

This only happens, when message is unread
Moved mail subject to new controller

PMMailHeaderViewController will take care of showing all necessary mail header
information
Constraints for HeaderView. Disabled header in MessageView
Update mailcore
Small UI adjustments
Selectable text in mail detail header