add a Mark as Unread menu item at the thread screen

app/src/main/kotlin/com/simplemobiletools/smsmessenger/activities/ThreadActivity.kt

@@ -199,6 +199,7 @@ class ThreadActivity : SimpleActivity() {
             R.id.block_number -> blockNumber()
             R.id.delete -> askConfirmDelete()
             R.id.manage_people -> managePeople()
+            R.id.mark_as_unread -> markAsUnread()
             else -> return super.onOptionsItemSelected(item)
         }
         return true
@@ -392,6 +393,16 @@ class ThreadActivity : SimpleActivity() {
         showSelectedContacts()
     }
 
+    private fun markAsUnread() {
+        ensureBackgroundThread {
+            conversationsDB.markUnread(threadId.toLong())
+            markThreadMessagesUnread(threadId)
+            runOnUiThread {
+                finish()
+            }
+        }
+    }
+
     @SuppressLint("MissingPermission")
     private fun getThreadItems(): ArrayList<ThreadItem> {
         messages.sortBy { it.date }