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/gtk/checkbox.cpp

@@ -105,6 +105,7 @@ bool wxCheckBox::Create(wxWindow *parent,
                         const wxValidator& validator,
                         const wxString &name )
 {
+    WXValidateStyle( &style );
     if (!PreCreation( parent, pos, size ) ||
         !CreateBase( parent, id, pos, size, style, validator, name ))
     {
@@ -112,11 +113,6 @@ bool wxCheckBox::Create(wxWindow *parent,
         return false;
     }
 
-    wxASSERT_MSG( (style & wxCHK_ALLOW_3RD_STATE_FOR_USER) == 0 ||
-                  (style & wxCHK_3STATE) != 0,
-                  wxT("Using wxCHK_ALLOW_3RD_STATE_FOR_USER")
-                  wxT(" style flag for a 2-state checkbox is useless") );
-
     if ( style & wxALIGN_RIGHT )
     {
         // VZ: as I don't know a way to create a right aligned checkbox with