refactor use string resources

app/src/main/res/layout/activity_main.xml

@@ -56,18 +56,18 @@
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:onClick="onButtonScrollLayoutClick"
-            android:text="ScrollLayout"/>
+            android:text="@string/scroll_view"/>
 
         <Button
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:onClick="onButtonImageSwitcherClick"
-            android:text="ImageSwitcher"/>
+            android:text="@string/image_switcher"/>
 
         <Button
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:onClick="onButtonOpenGLClick"
-            android:text="OpenGL"/>
+            android:text="@string/opengl"/>
     </LinearLayout>
 </ScrollView>

app/src/main/res/layout/activity_relative.xml

@@ -22,7 +22,7 @@
         <TextView
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            android:text="Layoutübersicht:"
+            android:text="@string/layout_uebersicht"
             android:id="@+id/textview_relative"/>
 
         <LinearLayout

app/src/main/res/values/strings.xml

@@ -6,8 +6,11 @@
     <string name="relative_layout">RelativeLayout</string>
     <string name="frame_layout">FrameLayout</string>
     <string name="grid_layout">GridLayout</string>
-    <string name="action_settings">Settings</string>
+    <string name="scroll_view">ScrollView</string>
+    <string name="image_switcher">ImageSwitcher</string>
+    <string name="opengl">GLSurfaceView</string>
 
+    <string name="action_settings">Settings</string>
     <string name="action_settings3D">Show 3D</string>
     <string name="action_settings2D">Show 2D</string>
     <string name="action_settings2DP">2D polar coordinates</string>