Minor UI improvements

- added outbox label
- notification is now always removed when on main activity
- send status is now shown in message list and detail view

app/src/main/res/drawable/sent_acknowledged.xml

@@ -0,0 +1,25 @@
+<!--
+  ~ Copyright 2016 Christian Basler
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~     http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+        android:width="24dp"
+        android:height="24dp"
+        android:viewportWidth="24.0"
+        android:viewportHeight="24.0">
+    <path
+        android:fillColor="#FF000000"
+        android:pathData="M18,7l-1.41,-1.41 -6.34,6.34 1.41,1.41L18,7zM22.24,5.59L11.66,16.17 7.48,12l-1.41,1.41L11.66,19l12,-12 -1.42,-1.41zM0.41,13.41L6,19l1.41,-1.41L1.83,12 0.41,13.41z"/>
+</vector>