Bump version to 1.3.

[mtias]

Changelog

• Cover image block supports an opacity range slider.
• Offer the option to convert a single block to an HTML block when conflicting content is detected.
• Persist recently used blocks through sessions.
• Added support for pasting plain text markdown content and converting to blocks.
• The block inspector groups features and settings in expandable panels.
• Accessibility improvements to the color palette component.
• Added a "feedback" link in the Gutenberg side menu.
• Use expandable panels for advanced block features (class name and anchor).
• Removed touch listeners from multi select.
• Added block descriptions to blocks that didn't have them.
• Allow stored values to be updated with new defaults.
• Refactor image block to use withApiData and fix issues with .tiff images.
• Clean up non inline elements when pasting inline content.
• Remove unused code in BlockList component.
• Added "transform into" text to block switcher.
• Fixed sidebar overflow causing extra scrollbars.
• Fixed multi-select inside new scroll container.
• Fixed image block error with .tiff image.
• Fixed the content overflowing outside the verse block container.
• Fixed issues with sticky quick toolbar position.
• Fixed hitting enter when a block is selected creating a default block after selected block.
• Fixed teaser markup in demo content.
• Clean working directory before packaging plugin.
• Updated Webpack dependencies.
• Updated Jest and React.