Opencart Patch for 2.2.x and below

This patch removes the barrier between old and new OpenCart versions allowing any developer to port their latest extensions for 3.x to older versions starting from 2.0.0.0.

For Opencart versions:
2.0.0.0; 2.0.1.0; 2.0.1.1; 2.0.1.1; 2.0.2.0; 2.0.3.1; 2.1.0.1; 2.1.0.2; 2.2.0.0

1. Adds support for multi folder structure like in opencart 2.3.x
2. Fixes calls to extension/dashboard/my_extension via ajax - removes permission block.
3. Adds a stable API in .../model/extension/d_opencart_patch to avoid compatibility issues between different opencart versions.
4. Uses the latest Opencart 3.x folder structure which allows you to port your 3.x extensions to older Opencart versions and install them via the default Extension/modules interface.