'Mark Read' instead 'Dismiss' notifications

'Dismiss' is a bit unclear - even though introduced at #2083 to make things clearer,
i do not think this has happened :) -
i also needed to dive into the source code to see what exactly 'Dismiss' is doing.

meanwhile, we're anyway using 'Mark Read' to remove unread counter from chats,
and this is basically the same. good to be consistent.
finally, this is also the term WhatsApp/Telegram are using.

closes https://support.delta.chat/t/what-does-dismiss-button-on-notifications-do/3261/3

nb: we also change the english 'short' term from 'Read' to 'Mark Read' -
this is sill shorter than many translations
and removes the ambiguousness 'Mark Read' vs 'Open for Reading" -
esp. in the notification without much context, this is better,
but also on iOS, where the term was introduced for, it is good to be explict.

src/main/java/org/thoughtcrime/securesms/notifications/NotificationCenter.java

@@ -452,7 +452,7 @@ public void addNotification(int accountId, int chatId, int msgId) {
                     PendingIntent markReadIntent = getMarkAsReadIntent(chatData, true);
 
                     NotificationCompat.Action markAsReadAction = new NotificationCompat.Action(R.drawable.check,
-                            context.getString(R.string.notify_dismiss),
+                            context.getString(R.string.mark_as_read_short),
                             markReadIntent);
 
                     if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {

src/main/res/values/strings.xml

@@ -56,7 +56,7 @@
     <string name="mark_all_as_read">Mark All as Read</string>
     <string name="mark_as_read">Mark as Read</string>
     <!-- Shortest text for "Mark as being read". In english, this could be "Read" (past tense of "to read"), in german, this could be "Gelesen". -->
-    <string name="mark_as_read_short">Read</string>
+    <string name="mark_as_read_short">Mark Read</string>
     <!-- Placeholder text when something is loading -->
     <string name="loading">Loading…</string>
     <string name="hide">Hide</string>
@@ -993,6 +993,7 @@
     <string name="mailto_link_could_not_be_decoded">mailto link could not be decoded: %1$s</string>
 
     <!-- notifications  -->
+    <!-- deprecated, use mark_as_read or mark_as_read_short -->
     <string name="notify_dismiss">Dismiss</string>
     <string name="notify_reply_button">Reply</string>
     <string name="notify_new_message">New message</string>