"Assign schedule" improvements

This MR contains a lot of improvements for scheduled operations.

• Core improvements:
• • Move all code that assigns a schedule for an operation to SKGOperationObject member;
• • Improve that code to handle all cases with template and non-template schedules;
• • Handle schedule unassign;
• UX improvements:
• • Add menu entry to unassign the schedule for an operation;
• • Make schedules ordered by date in the assign menu;
• • Visually separate active and inactive schedules;
• • Use same color for inactive schedules in the assign menu as in the scheduled operations view;
• • Mark current schedule(s) of selected operation(s) in the assign menu with bold font;
• Tests:
• • Extend tests for scheduled operations and cover new code;
• • Fix several bugs in tests code;
• Docs: added documentation and a screenshot for the "assign schedule" feature.