Use selected date of main window when pressing the add activity button

[lfleereviden]

The add activity button is often used when an activity of a past day is missing.
In such a cases, the day where a new activity should be added is already selected in the main window. So it would make much sence, that if the "add activity" button is pressed the dialog uses the selected date of the main windows instead of always using the current date.

[remast]

Sounds reasonable. Yet I don't really do a lot on the Java version, so you might want to try https://baralga-app.tack.dev/.