Make composer window not parented to main window

### Problem

Currently, the composer window is modal to the main window. This means that it is always in front of the main window. Also, when you activate either the main or composer window, the other is brought to the front as well. This is often undesirable.

### Proposal

Make the composer window a first-class window that is not modal or dependent on the main window.

### Prior Art (Optional)

This is how all other mail apps work, that I am aware of. This includes Outlook, Apple Mail, and Geary. In all of them, popped out messages and composers are first-class windows, not modal to the message list.