[NcModal] Remove `allowInModal` in favour of hotkey scoping similar to the focus trap

[skjnldsv]

Fine for a hotfix, but I'd propose to remove allowInModal in favour of hotkey scoping similar to the focus trap.

So that:

• It doesn't depend on a CSS class in the modal
• It supports not only NcModal but other modal popovers as well like NcPopover
• It supports nesting, for example, when a file picker dialog is open from the settings dialog, and each has its own hotkeys

Originally posted by @ShGKme in #7966 (review)