Close backends prior to shutdown

We need some way to signal the backends that we are about to shutdown,
allowing them to clean up (for example in notmuch committing the db changes).
This commit implements a hook which gets called upon shutdown, providing
backends implement the io.Closer interface.

aerc.go

@@ -178,4 +178,5 @@ func main() {
 			time.Sleep(16 * time.Millisecond)
 		}
 	}
+	aerc.CloseBackends()
 }

widgets/aerc.go

@@ -2,6 +2,7 @@ package widgets
 
 import (
 	"errors"
+	"io"
 	"log"
 	"net/url"
 	"strings"
@@ -405,3 +406,21 @@ func (aerc *Aerc) Mailto(addr *url.URL) error {
 	})
 	return nil
 }
+
+func (aerc *Aerc) CloseBackends() error {
+	var returnErr error
+	for _, acct := range aerc.accounts {
+		var raw interface{} = acct.worker.Backend
+		c, ok := raw.(io.Closer)
+		if !ok {
+			continue
+		}
+		err := c.Close()
+		if err != nil {
+			returnErr = err
+			aerc.logger.Printf("Closing backend failed for %v: %v\n",
+				acct.Name(), err)
+		}
+	}
+	return returnErr
+}