Use dedicated slots for handling exactly the actions they should handle

Previously, all actions in the header context menu were handled by the
same method slotHeaderContextMenuTriggered(), even actions in submenus
which was clearly not intended. For example, selecting sorting
By Date/Time of Most Recent in Subtree in the Sorting submenu did also
enable the Sender column (because the action for showing/hiding the
Sender column had the same index set as user data as the action for
sorting By Date/Time of Most Recent in Subtree). Usage of dedicated
slots attached to the actions' signals instead of a single slot
attached to the menu's triggered() signal (which is also emitted for
actions in submenus) fixes this.

BUG: 227516
FIXED-IN: 4.11.4
REVIEW: 113901