[plugins/mousepad]: Add support for the persistence feature of the RemoteDesktop portal

This allows us to avoid asking the user for permission for remote control on Wayland every time kdeconnectd is restarted for whatever reason (for example logging out or rebooting), at least in theory. The idea is that the SelectDevices call now also accepts a restore token, and if the user grants permission to persist a restore token will be returned in the response of the Start call.

Currently plasma/xdg-desktop-portal-kde!265 (merged) is required for this to work at all with Plasma 6, and even then persistence only works in the same session (for example if I restart kdeconnectd then I only get the notification instead of the permissions prompt), if I reboot the system then the token gets invalidated and the permissions dialog appears again, not sure if the issue is with what I'm doing here or if that's a bug in the portal.

Things that need to be checked:

• What happens if the portal implementation only has v1 of the protocol and not v2 (the one with persistence)?
  • In particular what happens for the SelectDevices call if a restore token is given despite the portal not supporting it
    • Seems fine with xdg-desktop-portal 1.14.4 at least
  • For the Start call we'll need to handle the case of the user denying the persistence request anyway
• Where and how should the restore token be stored?
  • I used KConfig just so I have something to test, but the restore token isn't really a setting
    • Updated to use KSharedConfig::openStateConfig
  • Most of KDE Connect's settings and data appear to be for each connected device
  • The device name is a global setting, but it's implemented using QSettings rather than KConfig, and currently only setName and getName is exposed in core/kdeconnectconfig.h

BUG: 479013