Add paste button

This adds a paste button to completion bar and menu popover to ease pasting text.

We can extend this to a clipboard manager easily as the objects already keep track of the last N copied elements and we just need to add the UI.

Let's get this already out though as it's useful on its own and we can already test the new protocol logic.

Demo: https://social.librem.one/@agx/113102801001329320