Calendar improvements

[david072]

This PR contains some small improvements for the calendar application:

• Added a date picker dialog to make selecting dates easier
• Refactored the code a little bit to make calendar events easier to work with
• Removed some unused code
• Ask about unsaved changes when attempting to close the window (fixes #21713)

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[ADKaster]

@david072 I rebased this and fixed the conflicts, and I'll merge once it passes CI. I did find a couple boogs that I'll open as issues though.