Support variables in written/coded workflows

### Real world problem:

As a developer I write the following example(!) workflow for a customer (not via UI):

> The customer likes to query the last 10 sold items in his store and lookup his purchase price for the items in a document. Then he likes to know how much he have earned.

I implemented it and all are happy.

...

- After 2 weeks the customer asks me if I can change the query to lookup the last 30 items he sold.
- A week later he would like to revert his decision and only get the last 10 items.
- Two months ahead and he has changed the name of the document with all the purchase prices and likes me to update the workflow.
- And so on

### Proposed solution

As a developer I would like to define properties in my workflow that the user can set via the UI. I would then implement the example workflow like this:

> The customer likes to query the last {{ ItemCount = 10 }} sold items in his store and lookup his purchase price for the items in a {{ DocumentName = "purchase_prices.xml" }}. Then he likes to know how much he have earned.

This way my customer can make simple changes by his own without my intervention. There should be an option menu in the workflow UI there the customer sees all available properties and there he can change the values of it.