switchUnits - Fix switching units not working (#6188)

[McDiod]

When merged this pull request will:

• Fix switchUnits functionality

---

Clicking on a switchable unit currently does nothing (see #6188).

It looks like BIS_fnc_addStackedEventHandler was changed at some point so that the arguments array is appended to onMapSingleClick's passed params, resulting in switchUnits failing here:

https://github.com/acemod/ACE3/blob/master/addons/switchunits/functions/fnc_addMapFunction.sqf#L24

Quick fix would have been:

params ["_pos", "", "", "", "_unit", "_sides"];
if (alive ACE_player && {GVAR(OriginalUnit) getVariable ["ACE_CanSwitchUnits", false]}) then {
    [[_unit,_sides], _pos] call FUNC(handleMapClick);
};

This replaces BIS_fnc_addStackedEventHandler with MapSingleClick mission eventhandler instead and introduces GVAR(switchableSides), because you can't pass arguments through addMissionEventHandler.