Customizer: add touch swiping to exit panels

[celloexpressions]

Originally proposed in #28784, along with some other touch gestures. We're probably going to take a different direction there, so let's address touch-gestures in the Customizer here instead. To start, we should allow users to move from a panel to the top-level controls by swiping right (which matches the horizontal-sliding animations for opening/closing panels well in terms of UX).

Need to figure out the best way to implement this - is it worth a touch events library, or should we just roll our own?

[designsimply]

Replying to celloexpressions:

is it worth a touch events library, or should we just roll our own?

Rolling your own would work best if you want to limit functionality, keep everything as fast as possible, and you are confident you will still be able to get all the functionality you need without resorting to a library. Would a library be used in other areas besides swiping to close panels such as the widgets panel?

[iseulde]

I don't think we can do this as this also means moving a page back and forward. See #31187.

[celloexpressions]

Going to this close for now; it is definitely possible to do this without interfering with browser navigation, but there doesn't seem to be much interest here at the moment. If revived in the future, keep in mind potential usefulness for large-screen touch devices, not just mobile.