Only remaining issue is that clicking on the titlebar of the window, to give it focus, is already interpreted as input. But clicking on the window in the task bar, or using alt tab works to get back, without causing an input event.
ControllerEmu::Control instances have a unique_ptr<ControlReference>
member, which is passed either an InputReference or OutputReference.
Without this virtual destructor, deleting a derived class through a
pointer to the base class is undefined behavior.
Better separation of concerns. Relegates `ControllerInterface` to
enumerating input controls, and the new `ControlReference` deals with
combining inputs and configuration expression parsing.
