screencast: Misc minor cleanups
The merge request a few commits of cleanups that shouldn't have any behavioural effect on the code, they're purely stylistic.
screencast: Drop unused headers
screencast: Convert event handlers into methods
ScreencastStream is a C++ class, but we're having C style event handling
as static functions within the class. Mixing two styles within a class
isn't very clear.
We can turn them into normal methods with some thin wrapper functions
round the event handling.
screencast: Use consistent prefix for member variables