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/DolphinWX/Frame.cpp

@@ -53,6 +53,7 @@
 #include "Core/HW/GCPad.h"
 #include "Core/HW/Wiimote.h"
 #include "Core/HW/WiimoteEmu/WiimoteEmu.h"
+#include "Core/Host.h"
 #include "Core/HotkeyManager.h"
 #include "Core/IOS/IOS.h"
 #include "Core/IOS/USB/Bluetooth/BTBase.h"
@@ -186,11 +187,11 @@ WXLRESULT CRenderFrame::MSWWindowProc(WXUINT nMsg, WXWPARAM wParam, WXLPARAM lPa
   case WM_USER:
     switch (wParam)
     {
-    case WM_USER_STOP:
+    case static_cast<int>(HostMessageID::WMUserStop):
       main_frame->DoStop();
       break;
 
-    case WM_USER_SETCURSOR:
+    case static_cast<int>(HostMessageID::WMUserSetCursor):
       if (SConfig::GetInstance().bHideCursor && main_frame->RendererHasFocus() &&
           Core::GetState() == Core::State::Running)
         SetCursor(wxCURSOR_BLANK);
@@ -758,7 +759,7 @@ void CFrame::OnHostMessage(wxCommandEvent& event)
   }
   break;
 
-  case WM_USER_CREATE:
+  case static_cast<int>(HostMessageID::WMUserCreate):
     if (SConfig::GetInstance().bHideCursor)
       m_render_parent->SetCursor(wxCURSOR_BLANK);
     if (SConfig::GetInstance().bFullscreen)
@@ -776,7 +777,7 @@ void CFrame::OnHostMessage(wxCommandEvent& event)
   }
   break;
 
-  case WM_USER_STOP:
+  case static_cast<int>(HostMessageID::WMUserStop):
     DoStop();
     break;