Handle editor width resize in InlineWidget

[njx]

Currently, each inline widget needs to manage its own width when the host editor resizes. We should provide reasonable default behavior for this which can be overridden. The default behavior in InlineColorEditor is probably what we want to use.

[peterflynn]

I wonder... the code in MultiRangeInlineEditor says we can't simply set the width to 100% because the DOM node has padding. But if we used box-sizing: border-box could we make it work? Sure would be a lot simpler than window-resize & document-change listeners...

[njx]

No, that's not the reason we have to set min-width manually. It's because setting it to 100% sets it to 100% of the scroller's clientWidth, not 100% of the scrollWidth. (Widgets aren't children of the lineSpace in our implementation--they're parented directly into the scroller.)

[njx]

Adding to sprint 18 since it's related to #2220 which I'm looking at.

[peterflynn]

NJ's changes have landed, so closing.

(Note also the confusion above about why 100% width doesn't work has been clarified in the docs, which are now in InlineWidget).