fix(drafts): Reveal or focus windows for drafts when already open

Fixes T3883

src/flux/stores/draft-store.coffee

@@ -407,6 +407,12 @@ class DraftStore
     title = if options.newDraft then "New Message" else "Message"
 
     save.then =>
+      app = require('remote').getGlobal('application')
+      existing = app.windowManager.windowWithPropsMatching({draftClientId})
+      if existing
+        existing.restore() if existing.isMinimized()
+        existing.focus()
+      else
         atom.newWindow
           title: title
           windowType: "composer"