Allow Facility Staff Slot UI class to be overridden depending on facility

[robojumper]

Currently, UIFacilityStaffContainer does this:

switch (Movie.Stack.GetCurrentClass())
{
case class'UIFacility_Academy':
	StaffSlots.AddItem(Spawn(class'UIFacility_AcademySlot', self).InitStaffSlot(self, LocationRef, i, onStaffUpdatedDelegate));
	break;
case class'UIFacility_AdvancedWarfareCenter':
	if (StaffSlot.IsSoldierSlot())
		StaffSlots.AddItem(Spawn(class'UIFacility_AdvancedWarfareCenterSlot', self).InitStaffSlot(self, LocationRef, i, onStaffUpdatedDelegate));
	else
		StaffSlots.AddItem(Spawn(class'UIFacility_StaffSlot', self).InitStaffSlot(self, LocationRef, i, onStaffUpdatedDelegate));
	break;
case class'UIFacility_PsiLab':
	if (StaffSlot.IsSoldierSlot())
		StaffSlots.AddItem(Spawn(class'UIFacility_PsiLabSlot', self).InitStaffSlot(self, LocationRef, i, onStaffUpdatedDelegate));
	else
		StaffSlots.AddItem(Spawn(class'UIFacility_StaffSlot', self).InitStaffSlot(self, LocationRef, i, onStaffUpdatedDelegate));
	break;
default:
	StaffSlots.AddItem(Spawn(class'UIFacility_StaffSlot', self).InitStaffSlot(self, LocationRef, i, onStaffUpdatedDelegate));
	break;
}

which is bad because it doesn't allow new Staff Slot types without major hackery using UIScreenListeners and also doesn't allow overrides of UIFacility classes.

A mechanism would be needed to expand this behavior to allow custom UIFacility_StaffSlot classes.

[robojumper]

My idea would be adding a property + getter to X2StaffSlotTemplate that returns the UI class for the staff slot. That way, we can break up this whole switch statement and replace it with something more sensible.

[robojumper]

Considerations: Adding a delegate / member to X2StaffSlotTemplate is a change in data layout, which is fine at the moment.
However, if it were to be made a native class in the future (not that I expect it to), this approach would break, in addition to mods that were to use it.
A safer option would be making it a function that reads from a config map, but can be overridden by subclasses.