- closes vtk/vtk#18951
- Refactored this method to just pop and dispatch events from the event
 queue.
- A new method `WaitForEvents` was introduced to monitor X display
 connections for user activity or until a timer wants to timeout.
- VTK will use `WaitForEvents` on X11 when the interactor is `Start()`ed.
- VTK will not use `WaitForEvents` on X11 when the interactor is asked
 to manually `ProcessEvents`.
- Updated documentation of ProcessEvents in all interactors.