New incoming pairing request ui in kcm

Quick motivation:
Currently a user accepts an incoming pairing request by clicking "accept" on a notification.
They should also check that the verification key on the notification matches the one on their phone etc, but although we show the key, we don't explain what it means; or what to do if it doesn't match.
We also don't explain what a pairing request does, or what to do with unsolicited requests (accepting gives effectively unrestricted, persistent access to a computer -- we don't want users accepting by reflex).

This MR:
This is a rough implementation of a new page for incoming pairing requests in the KCM.
Mostly, this just makes users confirm the verification key matches, then lets them pair; but it also explains what a pairing request and verification key do (with contextual help buttons).
If the user says the keys don't match; we precipitate a big yellow warning not to pair, and advice to avoid pairing on untrusted networks (although I'm sure there's more/better advice we could give here).

The incoming pairing request notifications from kdeconnectd then lead to this page, rather than pairing immediately.

Screenshots:

As it looks initially:

If user confirms keys match:

If user says keys don't match:

I'm sure it's quite clear that I've never done any UI before, so I won't mind if I'm told this is just irredeemably awful.
Many thanks.