Add conversion functions button type

[barnettwilliam]

It is not possible to invoke a conversion function via a button for example to convert between model text and an image and when testing PR #151 composite panels.

Currently only action functions can be used via the ActionButton class. There was the intention to add a class of button like Render button that specifies a renderfunction to use. This introduces additional terminology so it could just be a conversion function instead.

The workaround used in the YAMTL Playground is to define action functions in the tool for doing these conversions.