Module Multiple Layout Selection - Resubmit

While this modification may not benefit the default OpenCart modules, it will certainly help 3rd party developers and customers.

It allows the developer to use a check list of layouts that are stored in an array called "layouts" instead of a single drop down menu called layout_id.

This will greatly benefit customers when using 3rd party modules that support this system. Here's the difference:

Old Method
To display the same banner across multiple layouts, the customer has to create X instances of that module. 3rd party modules such as my Advanced Banners have over 40 different fields.

Using Multiple Layout Selection
To display the same banner across multiple layouts, a customer would simply select all the layouts that apply to that module instance, which means only a single instance has to be created.

Not only does this save the customer the headaches and frustrations of setting up third party systems, but it will also greatly reduce the number of module instances required to complete the same task that the current system supports.

Along with the added customer benefits it will also increase performance because the system has less module instances to go through when determining whether a module is valid or invalid. While it may not be a huge performance increase, every millisecond counts in large scale stores.

Another added benefit is reducing the likelihood of the customer reaching their PHP max_input_vars setting because it will reduce the number of post fields that are sent when saving the modules.

I urge you to reconsider the pull request as I just received another request for this feature from a customer. Please see his email below:

```
"Your Advanced Banner extension is working great for us. It's excellent.
We often put the same banner on multiple layouts. We may put a single banner on 6-8 different layouts.
It would be nice to use a single banner setup on all 8 layouts.
Is it possible to pick several layouts for a banner? So we don't have to make multiple copies of the same banners settings.
Please let me know if there is a fix for this."
```

Thanks,
Joel.