Increase Customizer Expand/Collapse Button for Accessibility

[mrwweb]

At present, the expand/collapse button for the customizer is 20px-square. That's half the minimum recommended target size for accessible touch interfaces.

The button should instead be 40px-square at a minimum and ideally 50px-square...er...circle. This would also hopefully begin to address the find-ability of the Customizer when collapsed raised in #29949.

[celloexpressions]

Increase padding on customizer collapse/expand button (no visual changes).

[celloexpressions]

We can fix this pretty easily by adding padding to the button element. This has no visual impact, but results in a click area 45px tall and fairly wide (varying with the translation text). See 36093.diff​.

[mrwweb]

Sounds like a good solution to me!

[ocean90]

In 37341:

Customize: Increase the target size of the expand/collapse button in the customizer.

To improve accessibility on touch devices increase the size of the button in the collapsed view from 24×28 to 44×45 pixel.

Props celloexpressions.
Fixes #36093.