[Backdrops] Fix text selection & right-clicks

[mj12albert]

Fixes #1705

• Adding WebkitUserSelect: 'none' to the inlined styles prevents text selection on touch

• Only main-button clicks on dialog backdrop should close it

• I have followed (at least) the PR section of the contributing guide.

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[mj12albert]

@atomiks I've only updated the dismiss behavior for Dialog so it doesn't close on right click (matches Radix)

Will leave the rest alone in this PR given the larger outside press issue

[atomiks]

Do you need regular userSelect for Android?

Also wonder if pointerEvents: undefined (instead of conditional rendering for openReason === 'hover' could be a problem

[mj12albert]

Also wonder if pointerEvents: undefined (instead of conditional rendering for openReason === 'hover' could be a problem

I tested Popover with a mouse and a phone and it seems ok, but openReason === 'hover' ? 'none' : 'auto' could work too, what do you think? @atomiks

[atomiks]

undefined should be ok