Move the message definitions to their own files.

Currently the CFWL messages are created by a macro in the CFWL_Message header
file. This Cl splits out the class to their own files and writes them directly
instead of using the macro.

Review-Url: https://codereview.chromium.org/2510823003
diff --git a/BUILD.gn b/BUILD.gn
index 847e47f..7b634a7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1183,7 +1183,18 @@
       "xfa/fwl/core/cfwl_listbox.h",
       "xfa/fwl/core/cfwl_listitem.cpp",
       "xfa/fwl/core/cfwl_listitem.h",
+      "xfa/fwl/core/cfwl_message.cpp",
       "xfa/fwl/core/cfwl_message.h",
+      "xfa/fwl/core/cfwl_msgkey.cpp",
+      "xfa/fwl/core/cfwl_msgkey.h",
+      "xfa/fwl/core/cfwl_msgkillfocus.cpp",
+      "xfa/fwl/core/cfwl_msgkillfocus.h",
+      "xfa/fwl/core/cfwl_msgmouse.cpp",
+      "xfa/fwl/core/cfwl_msgmouse.h",
+      "xfa/fwl/core/cfwl_msgmousewheel.cpp",
+      "xfa/fwl/core/cfwl_msgmousewheel.h",
+      "xfa/fwl/core/cfwl_msgsetfocus.cpp",
+      "xfa/fwl/core/cfwl_msgsetfocus.h",
       "xfa/fwl/core/cfwl_picturebox.cpp",
       "xfa/fwl/core/cfwl_picturebox.h",
       "xfa/fwl/core/cfwl_pushbutton.cpp",