This uses GetClipboardSequenceNumber() to detect clipboard
content changes. Clipboard Format Listener is a better way to
do this according to MS docs, but sequence number works and
the listener requires creating a dedicated thread and HWND
for monitoring, so I will save it for a later time.
This adds a clipboard monitoring API for backends which use polling
to monitor clipboard updates. --clipboard-monitor option can turn
clipboard monitoring on and off at runtime.
This adds a clipboard API with multiple backend and format support.
--clipboard-enable option can be toggled at runtime to turn native
clipboard on and off.
