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/Common/Common.h

@@ -57,13 +57,3 @@ struct CrtDebugBreak
 // Dummy macro for marking translatable strings that can not be immediately translated.
 // wxWidgets does not have a true dummy macro for this.
 #define _trans(a) a
-
-// Host communication.
-enum HOST_COMM
-{
-  // Begin at 10 in case there is already messages with wParam = 0, 1, 2 and so on
-  WM_USER_STOP = 10,
-  WM_USER_CREATE,
-  WM_USER_SETCURSOR,
-  WM_USER_JOB_DISPATCH,
-};