ControllerEmu, the class, is essentially acting like a namespace for
ControlGroup. This makes it impossible to forward declare any of the
internals. It also globs a bunch of classes together which is kind of a
pain to manage.
This splits ControlGroup and the classes it contains into their own source
files and situates them all within a namespace, which gets them out of
global scope.
Since this allows forward declarations for the once-internal classes, it
now requires significantly less files to be rebuilt if anything is changed
in the ControllerEmu portion of code.
It does not split out the settings classes yet, however, as it
would be preferable to make a settings base class that all settings derive
from, but this would be a functional change -- this commit only intends to
move around existing code. Extracting the settings class will be done in
another commit.
Better separation of concerns. Relegates `ControllerInterface` to
enumerating input controls, and the new `ControlReference` deals with
combining inputs and configuration expression parsing.
ControllerEmu is a massive class with a lot of nested public classes.
The only reason these are nested is because the outer class acts as a
namespace. There's no reason to keep these classes nested just for that.
Keeping these classes nested makes it impossible to forward declare them, which leads to quite a few includes in other headers, making compilation take
longer.
This moves the source files to their own directory so classes can be
separated as necessary to their own source files, and be namespaced under the
ControllerEmu namespace.
