[1.4.8] Add a new dashboard widget to display the total amount of assets

Currently, if some accounts are hidden from the dashboard, it is not possible to see your total assets at a glance. Instead, you have to manually show all accounts and then hide them afterwards. To avoid this, a new widget should be added to the dashboard that will always (regardless of whether an account is hidden or not) display the total assets including the trend for the previous period, so you can easily keep an eye on them.