- Added `InlineWidgetContainerDefaults` for deduping inline widget code
- Added a helper function `WidgetSpan.extractFromInlineSpan` for extracting `WidgetSpan`s and automatically applying text scaling (at widget level)
- Removed `TextPainter.inlinePlaceholderScales`. I'm going to deprecate the `scale` argument in `TextPainter.addPlaceholder` next, as scaling is now done at the widget level.
- Added runtime check and comments to make sure nobody is extending `PlaceholderSpan` directly (unfortunately we can't remove `PlaceholderSpan` without moving RenderEditable and RenderParagraph to the widgets library).
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