nicer preview of decrypt_text_fragment layout

OpenKeychain/src/main/res/layout/decrypt_text_fragment.xml

@@ -1,11 +1,13 @@
 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:orientation="vertical">
 
     <LinearLayout
         android:visibility="gone"
+        tools:visibility="visible"
         android:id="@+id/decrypt_content"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
@@ -28,6 +30,7 @@
                 android:layout_height="wrap_content"
                 android:gravity="top"
                 android:hint=""
+                tools:text="This is the plaintext"
                 android:textIsSelectable="true" />
 
         </ScrollView>