Config system v1

Roughly inspired by what I've learned from my work on Space Station 14, without some of the unnecessary cruft and complexity.

Implementation is relatively simple once I figured out all the template order shenanigans.

src/d/d_camera.cpp

@@ -766,7 +766,7 @@ void dCamera_c::updatePad() {
         var_f31 = mDoCPd_c::getSubStickX3D(mPadID);
 
         #if TARGET_PC
-        if (dusk::ImGuiMenuEnhancements::m_enhancements.invertCameraXAxis) {
+        if (dusk::settings::enhancements::InvertCameraXAxis.getValue()) {
             var_f31 *= -1.0f;
         }
         #endif