Make it possible to access template renderer parameter values

[Legioth]

When creating a UI unit test for a Grid that uses a template renderer (i.e. LitRenderer or the deprecated TemplateRenderer), it's useful to find withProperty values that were assigned for a specific cell.

As an example, I might have a column defined like this:

grid.addColumn(LitRenderer.<Person>of("Name: ${item.name}").withProperty("name", Person::getName));

For this grid, I would want to assert that the value of name for a specific row is e.g. "John Doe".