Maya Tool - Solver UI - Add a Refresh button rather than using Callbacks

[david-cattermole]

When adding attributes to the Solver UI window, there is a lag. The cause is Maya Callbacks. This is being addressed in #18, however until that is fixed and Callbacks become stable, we should make a "Refresh" button the user can press to update the UI.

The "Refresh" button and Callbacks are not needed together, but we should enable the option to use Callbacks, if the user wants to. The Callbacks will be turned off by default.
Therefore we should add a module configuration option to turn this feature on/off.

For testing and development, it is recommended to turn on Callbacks, so we find and refine bugs.

[david-cattermole]

Progress has been made on #18.
We probably don't need a "refresh" button.