Add sleep tracking, statistics module and backup features

Features:
- Sleep tracking with timer and manual duration input
- Statistics module with 5 tabs (daily summary, feeding, diapers, sleep, growth)
- Export/Import backup functionality in settings
- Complete German, French and Italian translations

app/src/main/AndroidManifest.xml

@@ -30,6 +30,10 @@
             android:name=".SettingsActivity"
             android:label="@string/settings_title"
             android:theme="@style/Theme.LunaTracker"/>
+        <activity
+            android:name=".StatisticsActivity"
+            android:label="@string/statistics_title"
+            android:theme="@style/Theme.LunaTracker"/>
     </application>
 
 </manifest>