Allow only a single inline composition at a time

With this, we introduce a dialog when you would try to add another.  We
also use this when changing the selected conversation with a composer
open.
Compose new messages inline, with no conversation selected
Hook up composer accelerators to main window when composer is embedded

It would be nice to only activate these accelerators when the composer
has focus, but that doesn't seem to be easy to detect.
Hook up composer accelerators only when focus is in composer editor