Detect attempts to catch wxEVT_TEXT_ENTER without wxTE_PROCESS_ENTER

This is never going to work, so complain about trying to do it to help with
catching this bug.

This is possible thanks to the new OnDynamicBind() method invoked whenever a
dynamic event handler is bound to a control, so this doesn't detect all
possible occurrences of the bug (such as specifying the handler in a static
event table or in a validator), but it's still better than nothing.

In the future OnDynamicBind() should be extended for other invalid calls, e.g.
binding a handler for wxEVT_TEXT_ENTER to a non-text control shouldn't work
neither, ideally.

include/wx/textctrl.h

@@ -743,6 +743,11 @@ public:
     }
 
 protected:
+    // Override wxEvtHandler method to check for a common problem of binding
+    // wxEVT_TEXT_ENTER to a control without wxTE_PROCESS_ENTER style, which is
+    // never going to work.
+    virtual bool OnDynamicBind(wxDynamicEventTableEntry& entry);
+
     // override streambuf method
 #if wxHAS_TEXT_WINDOW_STREAM
     int overflow(int i) wxOVERRIDE;