- Added to main menu possibility to export and import all application date.

- Improved encryption/decryption of exported/imported data.  If accounts
  were exported, they need to be exported again.
- Lots of other implemetation improvements.

app/src/main/res/menu/main_menu.xml

@@ -6,6 +6,12 @@
   <item android:id="@+id/accounts"
       android:title="@string/accounts" />
 
+  <item android:id="@+id/export_all"
+      android:title="@string/export_all" />
+
+  <item android:id="@+id/import_all"
+      android:title="@string/import_all" />
+
   <item android:id="@+id/about"
         android:title="@string/about" />