Fixes applied:

1. TabWidget and KeyTreeView were saving sorting preset to the config group TabWidget::setKeyFilter with overriding settings of each other.

In other words all tabs were saved to the same group.
I've fixed it by setting config group to View #1, 2, 3... as it should be.
I've also had to prevent saving empty filter of last tab.

2. TabWidget didn't save settings on exit - added call to saveViews() to destructor
3. Prevented saving empty filter for the last tab in TabWidget::setKeyFilter()

Steps:

1. Create several tabs with different filters
2. Apply sorting to some columns
3. Exit from kleopatra
4. Run kleopatra

Expected result:
List of tabs, filters and sorting should be restored after app restart