Create a variable-template

[SimonBiggs]

Create a "variable-template" or something similar that takes a python string as input. The contents of that python variable are written into the form as part of the template.

This will enable shipping custom toolbar buttons as packages and similar.

It will also enable template reuse, and other interesting use cases.

[SimonBiggs]

Have a ScriptedForms function that can be called by python to append items to the template.