kcm: Copy model data into the "Forget…" dialog

…and refactor it into dynamic component. New required properties and a signal make it completely decoupled from surrounding context.

Future directions:

1. Theoretically, now we could save even more memory and loading times by factoring it out in a separate file.
2. Dialog needs to be closed in cases where its device somehow disappears or adapter is disconnected.

BUG: 459855
FIXED-IN: 5.26

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actually, no idea who should be added for reviews. fell free to CC people if needed.