XEH - Add support for backpack containers

[commy2]

When merged this pull request will:

• add XEH support for backpack containers
  spoiler

  "title"

class Extended_InitPost_EventHandlers {
    class Bag_Base {
        Mission_Test = "systemChat str _this; diag_log _this";
    };
};

Note however, that the retroactive flag does not seem to pick up already existing backpacks:

["Bag_Base", "InitPost", {
    systemChat str _this;
}, nil, nil, true] call CBA_fnc_addClassEventHandler;

[commy2]

For the problem above, I don't see a simple work around.

allMissionObjects "" apply {everyBackpack _x}

would be very slow.

[PabstMirror]

Doesn't fire for backpacks added to cargo: car addBackpackCargo ["B_AssaultPack_mcamo_Ammo", 1]
or backpacks from disassembled static weapons