Drag and drop emails into folders

## Problem
On the new Mail 2.0, I cannot move an email message to a specific folder.

The use case:
I keep my inbox as a "to-do list", and after handling an email, I move it to its appropriate folder. For example, if an email is a newsletter, after reading it I move it to the "Newsletter" folder, where it remains archived. On Mail 1.0 I used to be able to drag and drop the emails to the folders, which is not possible anymore with 2.0. Also, I could not find any other way of moving to a specific folder, only archiving.

## Proposal
I believe simply restoring the functionality that was in 1.0 will suffice here. A "Move To" button on the headerbar would be a nice first step, and drag-and-drop support would be even better.

Thanks 😄