Add changelog & version bump to 1.3.0

[ryelle]

What it says. Currently this is incrementing to 1.3.0-RC, so I can use this branch to tag an RC release today. Tomorrow, we'll bump the version again, check over the readme, and merge this prior to tagging the official 1.3.0 release + pushing to wp.org.

How to test the changes in this Pull Request:

1. Read over the readme.txt
2. View the plugin in your plugins list, it should read version 1.3.0
3. Expect: It shouldn't appear as version 1.2 anywhere
4. Expect: No other documentation is needed

[ryelle]

@LevinMedia Did you want to edit the readme at all? We added this for 1.2.0, do you want to swap out something for 1.3.0, or let the changelog be enough?

New for 1.2.0: We’ve added a stand-alone Product Category block to simplify the experience and improve the category search and selection UI. Be on the lookout for additional stand-alone blocks in future releases.

[ryelle]

Maybe we should remove the screenshots of the legacy block, too

[LevinMedia]

👍