ControlGroup: Convert group type enum into an enum class

Gets some constants out of the ControllerEmu namespace, and modifies
ControlGroup so that it uses the enum type itself to represent the
underlying type, rather than a u32 value.

Source/Core/InputCommon/ControllerEmu/ControlGroup/MixedTriggers.cpp

@@ -17,7 +17,7 @@
 namespace ControllerEmu
 {
 MixedTriggers::MixedTriggers(const std::string& name_)
-    : ControlGroup(name_, GROUP_TYPE_MIXED_TRIGGERS)
+    : ControlGroup(name_, GroupType::MixedTriggers)
 {
   numeric_settings.emplace_back(std::make_unique<NumericSetting>(_trans("Threshold"), 0.9));
 }