Cleaning up submit-settings.php

The most recent problems we've had with Dropplets so far have been with the `submit-settings.php` file. It contains a bit of *meta-PHP*, whereby we're representing data (the settings) as code, and we've written code which writes code.

In order to maintain backwards compatibility, I've kept it largely how it is. However, I've made it easier to modify and kept the manual PHP writing to a minimum. However, I recommend this be changed to a hash map ...

``` php
$settings[$key] = $value;
```

... in a future release. Since otherwise templates will be relying on static variables rather than something dynamic like an array/tree etc.