Support default value in macro prompt

[warmans]

1. Adds an optional third value to prompts that contains the default value e.g. {prompt|pos|0} would show 0 in the input labeled pos when the panel opens.
2. Updates help text to describe how to set defaults (also adds note about reusing placeholder values).
3. Removes some unused imports.

[breiler]

Clean and simple. Can you just add a couple of <br/> in the the helper text as it gets quite long at the moment:

[warmans]

Hi, thanks for reviewing. I've tidied up the text a bit and put in some breaks. Let me know if there are any other changes needed.