EditableText: support additional editing customisation [feature request]

It would be convenient to have more control around the edit process.

My motivating example is to differentiate display-value vs edit-value; for example display a formatted currency string "$1,234.50", but present the raw number (1234.5) for editing.

I can think of a few possibilities off the top of my head to implement this:

• an optional format function
• different properties for display and editing values
• a callback fired before editing starts
• (more involved) a different component for editing, meaning you could also use a select list on edit, or a colour picker, etc

I'm not sure which might be the best solution, if this is desirable functionality, but I'm happy to work on a patch for feedback if necessary.