Add missing button titles

[aamct2]

Issue: #6123

What I did

• Added titles to some of the image buttons that were missing them. This helps to meet WCAG 2.1 (Level A) for accessibility.
  • WCAG 2.1 Understanding Success Criterion 4.1.2: Name, Role, Value
  • Deque University's guidance on buttons having discernible text

How to test

Run any of the examples and inspect the adjusted buttons to see that they now have a title attribute specified. These issues can also be picked up by Lighthouse in the Accessibility audit in the Chrome Developer Tools.

[Armanio]

Good job @aamct2
One notice: when you update UI you must regenerate snapshots for test passes.
More information you can read here.
When the snapshots will be updated i will merge PR.
Could you update the snapshots?

[codecov]

Codecov Report

Merging #6124 into next will decrease coverage by 0.49%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             next    #6124     +/-   ##
=========================================
- Coverage   38.07%   37.57%   -0.5%     
=========================================
  Files         643      643             
  Lines        9414     9407      -7     
  Branches     1370     1341     -29     
=========================================
- Hits         3584     3535     -49     
- Misses       5265     5307     +42     
  Partials      565      565

Impacted Files	Coverage Δ
lib/ui/src/components/sidebar/SidebarSearch.js	63.63% <ø> (ø)	⬆️
lib/ui/src/components/sidebar/SidebarHeading.js	88.88% <ø> (ø)	⬆️
lib/ui/src/components/preview/preview.js	86.44% <50%> (-1.5%)	⬇️
addons/cssresources/src/constants.ts	0% <0%> (-100%)	⬇️
addons/cssresources/src/css-resource-panel.tsx	0% <0%> (-100%)	⬇️

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[Armanio]

Thanks!