Indicate unsaved work in red close button

Summary

Display the dark dot in the red close button to indicate unsaved work. See #162 and #163.

[x] Indicate when a rating changed in Ratings dialog.
[x] Dialog to save ratings when quitting with unsaved changes.
[x] Indicate when availabilities changed.
[x] Dialog to save availabilities when closing with unsaved changes.
[x] indicate when a ticket status changed.
[x] Indicate when an attendance changed.
[x] Indicate when a rating changed in Screening dialog.
[x] Dialog to save screening info when closing Screening Info dialog with unsaved changes.
[x] Indicate when state changes by key equivalents/menu items.
[x] Dialog to save screening info when closing Screens dialog with unsaved changes.
[x] Hide indicator when all changes are saved.
[x] Check for windows with unsaved changes when quitting.
[x] Update documentation.