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
(cherry picked from commit aa51b5e4)