Don't send EVT_TEXT_ENTER to controls without wxTE_PROCESS_ENTER

wxMSW always sent this event to multiline text controls, even when they
didn't have wxTE_PROCESS_ENTER style, contrary to what was documented.

Avoid sending this event unless wxTE_PROCESS_ENTER is used and add unit
tests checking that multiline text controls don't get it without this
style (but still do get it with it).

docs/changes.txt

@@ -73,6 +73,11 @@ Changes in behaviour not resulting in compilation errors
 - wxDC::DrawCheckMark() draws the same shape under all platforms now, use the
   new wxRendererNative::DrawCheckMark() to draw MSW-specific themed check mark.
 
+- wxTE_PROCESS_ENTER must be used to receive wxEVT_TEXT_ENTER events from even
+  multiline wxTextCtrl, conforming to the documentation, but contrary to the
+  previous behaviour in wxMSW, when these events were always generated in this
+  case. Please add wxTE_PROCESS_ENTER style if you relied on the old behaviour.
+
 
 Changes in behaviour which may result in build errors
 -----------------------------------------------------