[server] Add a method IdleInterface::simulateUserActivity

Summary:
So far only the client was able to simulate user activity. This new
method allows the server to also simulate user activity on all created
idle timeouts. This is required by KWin to prevent idle timeouts when
the user interacts through KDE Connect's virtual touchpad. In that
situation the mouse pointer is used without updating the input time
stamp as it doesn't come from "real" input devices and thus the idle
timeout prevention is not activated.

Reviewers: #frameworks, #plasma, #kwin

Subscribers: plasma-devel

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D9510