Create CFWL_EvtEditChanged

This CL creates a generic event for CFWL_EventType::EditChanged and uses it
in the ComboBox and DateTimePicker. None of the params from the custom events
were used so they have been removed.

Review-Url: https://codereview.chromium.org/2511813002
diff --git a/BUILD.gn b/BUILD.gn
index 5666168..f04ec48 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1188,6 +1188,8 @@
       "xfa/fwl/core/cfwl_evtclick.h",
       "xfa/fwl/core/cfwl_evtclose.cpp",
       "xfa/fwl/core/cfwl_evtclose.h",
+      "xfa/fwl/core/cfwl_evteditchanged.cpp",
+      "xfa/fwl/core/cfwl_evteditchanged.h",
       "xfa/fwl/core/cfwl_evtkey.cpp",
       "xfa/fwl/core/cfwl_evtkey.h",
       "xfa/fwl/core/cfwl_evtkillfocus.cpp",