InlineEditor should support default text

Use case:

Product.programminglang can be None when nothing has been set yet. When showing the programming languages and allowing inline editing, I want to display "Not yet specified" as the default text. As soon as the edit button is clicked though, I want the edit field to be cleared. If the edit is canceled, the widget should display "Not yet specified" again. If the field has a value, either because it's initial value was not None or because the user edited it inline, then the value should be shown as the text.

Here's some thoughts captured from IRC:

<mars> barry, ok, so you want to decouple the inline editor from the on-page text, if the on-page text is the default value
<mars> barry, should be simple: add an attribute "defaultValue" to the EditableText (not the inline editor)
<mars> barry, and then you would certainly need to work with the show/hide state changes, or set the default editor state
<mars> barry, I think setting the default editor state may be cleaner, and then on hide, check the editor's value - if it is 'null', show the default text again
<mars> barry, well, null or ""
<barry> mars: that sounds about right
<barry> mars: although it also sounds like enough work to not want to do it in this branch ;0
<mars> null or "" or s.replace(' ', '')
<mars> barry, true, but if you do write a new branch, it should be small
<mars> and cohesive
<barry> mars: right. it's a lazr-js change though so it would have to be separate anyway
<mars> barry, make that "defaultText", not "defaultValue" - you want to show the defaultText if the editor has the defaultValue :)