Refactor PairingHandler
We now have a single PairingHandler for all types of links.
The implementation now is more aligned with the one for Android and naming is more consistent (eg: "trusted" was renamed to "pairing" which was mixed before).
The new code also lets us cancel pairing requests after sending them (BUG: 469877), which was already fixed on Android.
Equivalent MR for Android: kdeconnect-android!367 (merged)
Edited by Albert Vaca Cintora