clean up the layout & polished Mensa credit dialog

* don't crash if the nfc tag has been removed between triggering intent and connecting to the tag
* closes #21

app/src/main/res/layouts/dialogs/layout/dialog_mensa_credit.xml

@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+        android:id="@+id/linLayout_mensacredit"
+        android:layout_width="wrap_content"
+        android:layout_height="match_parent"
+        android:orientation="vertical">
+
+    <TextView
+            android:id="@+id/txtView_Heading"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:text="@string/mensa_credit"
+            android:textAlignment="center"
+            android:textSize="24sp"
+            android:textStyle="bold" />
+
+    <TextView
+            android:id="@+id/txtView_current"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:layout_marginTop="7dp"
+            android:layout_marginBottom="7dp"
+            android:text="16,95 €"
+            android:textAlignment="center"
+            android:textSize="20sp" />
+
+    <TextView
+            android:id="@+id/txtView_last"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:text="@string/mensa_last"
+            android:textAlignment="center"
+            android:textSize="16sp" />
+</LinearLayout>