Re-enable user-select for dialogs

[oleq]

📝 Provide a description of the improvement

ATM the dialog component overlay has user-select: none. It has been added during dialog system development to prevent accidental selection when the modal is displayed. It also prevents the user from starting a selection in a dialog and then slipping into editing root, which is handled correctly by the browser but may be distracting.

In reality, this brings more harm than good, though. Features such as Track Changes Preview or Accessibility help set user-select: text on their dialogs to allow basic interactions (e.g. it's logical to copy some of their content, which isn't possible with selection disabled). Regular dialogs such as Media embed should also allow users to select fields' labels (e.g., because they may want to copy and translate them). Blocking selection is a UX anti-pattern here too.

I think that "whitelisting" features one by one is pointless and I propose we leave user-select: none on .ck.ck-dialog-overlay but we re-enable it on .ck.ck-dialog. Otherwise, this will become unmaintainable down the road.

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