This adds some built-in actions for handling hint selections without
having to spawn external applications.
The new actions are `Copy`, `Select` and `Paste`.
This adds support for hints, which allow opening parts of the visual
buffer with external programs if they match a certain regex.
This is done using a visual overlay triggered on a specified key
binding, which then instructs the user which keys they need to press to
pass the text to the application.
In the future it should be possible to supply some built-in actions for
Copy/Pasting the action and using this to launch text when clicking on
it with the mouse. But the current implementation should already be
useful as-is.
Fixes#2792.
Fixes#2536.
Christian Duerr