Input Capture support

Input capture is need to support applications like [input-leap](https://github.com/input-leap/input-leap) , and [deskflow](https://github.com/deskflow/deskflow) (formally synergy).

Support has added in the KDE and GNOME portals.
I am raising this issue in portals without support.  

The portal should also allow input capture access to the clipboard. 