fixes #2983

* Fix all the little RTL layout bugs

- [ ] borders on toolbox and workspace mirror correctly
- [ ] spacing on stage selector is correct
- [ ] stage size icons are mirrored
- [ ] extension library coming soon text is positioned correctly
- [ ] only mirror blocks icon on the tab (others stay right-handed)
- [ ] drop down icon in create variable modal has correct spacing

* FIx RTL backpack header border-radius

* round the bottom corner of paint/sound editors

This makes the editor’s asset panels match the blocks.

This bled into normalizing z-indices for the action menus in the z-index.css. The tooltip id was not unique between action menus for ‘suprise’ and ‘paint’, so `mainTooltipId` was used as a prefix to make them unique.

- Auditing color CSS and componet CSS to make consistent.
- Made a first pass of changes to increase contrast to elements that previously were problematic

Add a jest unit test for making sure the number is rendered out when
provided and not when not provided.

Add duplicate button. Pass the costume ID to paint (the previous costume ID used matches when duplicate is used.) When switching to the costume tab, open the current costume.

* Implement paint-75: Add blank costume button

* add backdrop icon, use default name

thanks @fsih !

This is for https://github.com/LLK/scratch-paint/pull/194, making the selector area smaller on tablet so as to create more space for the paint editor

Keep localization in the container level. Ideally we should not be passing message descriptors.

Costumes and Sound tab strings.