OSC-5522[^1] gives the terminal application access to all offered
mime-types when reading the clipboard, and allows setting multiple
mime-types when writing to the clipboard (with different contents, if
it so wishes).
In addition to the base protocol, we also implement the _event
extension_[^2], where a paste action (e.g. ctrl+shift+v) results in an
unsolicited mime-type listing being sent to the terminal
application (as if it had issued a mime-type query itself), instead of
the clipboard content being pasted directly. The application follows
up with an explicit read request (or chooses to ignore the event).
The protocol supports "passwords", as a way of bypassing terminal
popups asking the user for approval, after the first popup has been
approved by the user. Foot doesn't implement this kind of user
approval, but all read and write requests are denied with EPERM if the
user has disabled OSC copy/pasting with the security.osc52
configuration option. In addition, if disabled, event reporting cannot
be enabled at all (i.e. 'CSI ? 5522 h' is ignored).
[^1]: https://sw.kovidgoyal.net/kitty/clipboard/
[^2]: https://rockorager.dev/misc/bracketed-paste-mime/
* action() returns void - this gets rid of checks in vt_from_slave()
* split up ACTION_PRINT into ACTION_PRINT (ASCII) and ACTION_UTF8_PRINT
ACTION_PRINT is on the hot path, and we want it streamlined.
* Remove run-time checkout for unimplemented state transitions, as we
shouldn't have any of those left.
* Don't re-load current VT state on each iteration in vt_from_slave()
