v1.13.0: Interval reminders

[Futsch1]

In v1.13.0, a new type of reminder was introduced: Interval reminders. They are triggered every n minutes, hours or days, either starting from an initial point in time or from the time when the previous reminder was marked taken or skipped. These are somewhat similar to the linked reminders, only that they do not need a time based reminder to start them.

To create these new reminders, the UI to create reminders was completely overhauled. Instead of popping up two dialogs in a row (for amount and time), now a single dialog is shown containing all the required settings:

Furthermore, the required SDK (and therefore Android) version was lowered to 28 (Android 9) to make the app available to a much bigger number of devices (thanks to @wh201906 for your support).

As two minor changes, the statistics graph UI was improved a bit now showing a border around bars. The statistics view in general was changed to now show all three aspects of statistics (graph, table and calendar) on an equal menu level and saving the last active view.