[WIP] Core now adds a ~ by itself

Companion for
deltachat/deltachat-core-rust#6411. This PR is not
meant to change user-visible behavior, we just shifted the
responsibility to add the "~" to core. (without this PR, the name would
be "~~Alice" sometimes)

src/main/java/com/b44t/messenger/DcMsg.java

@@ -168,7 +168,7 @@ public JSONObject     getWebxdcInfo      () {
     public String getSenderName(DcContact dcContact) {
         String overrideName = getOverrideSenderName();
         if (overrideName != null) {
-            return "~" + overrideName;
+            return overrideName;
         } else {
             return dcContact.getDisplayName();
         }