Better separation of concerns. Relegates `ControllerInterface` to
enumerating input controls, and the new `ControlReference` deals with
combining inputs and configuration expression parsing.

There is no reason to prevent the user from closing the config dialog
if the device is not found. It's not very good UX…

Also fixes ExpressionParser to return NO_DEVICE if the device doesn't
exist instead of SUCCESS.

This adds RemoveDevice() to ControllerInterface, fixes ExpressionParser
and some other code to support device removals without crashing,
and adds an IsValid() method to Device, to prepare for hotplugging.

All affected files use shared_ptr but do not include memory which breaks
building without precompiled headers.

Small cleanup by using std::shared_ptr and getting rid of
ciface.Devices() which just returned the m_devices (which defeats the
point of making m_devices protected).

Incidentally, this should make the code safer when we have
different threads accessing devices in the future (for hotplug?).

A lot of code use Device references directly so there is
no easy way to remove FindDevice() and make those unique_ptrs.