Common: Move host communication enum to Host.h

Given this is actually a part of the Host interface, this should be
placed with it.

While we're at it, turn it into an enum class so that we don't dump its
contained values into the surrounding scope. We can also make
Host_Message take the enum type itself directly instead of taking a
general int value.

After this, it'll be trivial to divide out the rest of Common.h and
remove the header from the repository entirely

Source/Core/Core/Core.h

@@ -100,7 +100,7 @@ void UpdateWantDeterminism(bool initial = false);
 void QueueHostJob(std::function<void()> job, bool run_during_stop = false);
 
 // Should be called periodically by the Host to run pending jobs.
-// WM_USER_JOB_DISPATCH will be sent when something is added to the queue.
+// WMUserJobDispatch will be sent when something is added to the queue.
 void HostDispatchJobs();
 
 void DoFrameStep();