Added: Allow modules to override module admin and front controllers

[abhishek-webkul]

This PR allows modules to override admin and front controllers of existing modules.

Following examples are for module mydemomodule:

1. Overridding module admin controller

To override admin controller AdminAddHotelController of module hotelreservationsystem you need to create file mydemomodule/override/modules/hotelreservationsystem/controllers/admin/AdminAddHotelController.php

Override class AdminAddHotelController as:

class AdminAddHotelControllerOverride extends AdminAddHotelController
{
    // define overridden properties, constants and/or methods
}

1. Overridding module front controller

To override front controller Payment of module bankwire you need to create file mydemomodule/override/modules/bankwire/controllers/front/payment.php

Override class BankwirePaymentModuleFrontController as:

class BankwirePaymentModuleFrontControllerOverride extends BankwirePaymentModuleFrontController
{
    // define overridden properties, constants and/or methods
}