Improve validation of wxCheckBox styles.

Detect when incompatible styles are used (this required changing the value of
wxCHK_2STATE to be non-null) and sanitize the styles (after asserting) in this
case.

Put the validation code in wxCheckBoxBase instead of having slightly different
versions of it in port-specific wxCheckBox implementations.

Add a unit test checking that the expected asserts are indeed generated.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65824 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/checkbox.cpp

@@ -153,22 +153,16 @@ bool wxCheckBox::Create(wxWindow *parent,
 {
     Init();
 
+    WXValidateStyle(&style);
     if ( !CreateControl(parent, id, pos, size, style, validator, name) )
         return false;
 
     long msStyle = WS_TABSTOP;
 
     if ( style & wxCHK_3STATE )
-    {
         msStyle |= BS_3STATE;
-    }
     else
-    {
-        wxASSERT_MSG( !Is3rdStateAllowedForUser(),
-            wxT("Using wxCH_ALLOW_3RD_STATE_FOR_USER")
-            wxT(" style flag for a 2-state checkbox is useless") );
         msStyle |= BS_CHECKBOX;
-    }
 
     if ( style & wxALIGN_RIGHT )
     {