Re-enabled write mode in KeePassLib

Set database as part of App (so, Singleton)
Changed main page view model
WIP and testing saving mechanism

ModernKeePass/Pages/GroupDetailPage.xaml

@@ -77,6 +77,8 @@
                             </DataTemplate>
                         </ListView.ItemTemplate>
                     </ListView>
+
+                    <Button x:Name="addGroupButton" VerticalAlignment="Bottom" HorizontalAlignment="Right" Click="addGroupButton_Click" Content="Add Group" />
                 </StackPanel>
             </GridView.Header>
             <GridView.ItemContainerStyle>