Add variable example value to increase user understanding

[JeanMertz]

This would improve the story around variables, similar to #1 and #3.

Let's extend variables:

struct Variable {
    key: String,
    description: Option<String>,
+   example_value: Option<String>,
}

Which allows us to turn the current situation:

Into one where we have an extra (optional) tool in our toolbox to make the purpose of the variable as clear as possible to the user:

In this case, the example_value is set to hello world, and is shown as e.g. "hello world". If no example value is set, the field is kept empty.

If a default value is set (see #3), it takes precedence, and no example value is shown (unless the default value is removed, and an empty field is left, in which case the UI would put back the example value).

Note that the above example uses the placeholder attribute, so it's not actually a value that can be used as the input value.

[JeanMertz]

This has been implemented in 5c9788b and a57d0ab.