Make wxGridCellAttr ctor taking a single argument explicit

There doesn't seem to be any reason for allowing to implicitly convert
wxGridCellAttr pointer to wxGridCellAttr object.

docs/changes.txt

@@ -146,6 +146,8 @@ Changes in behaviour which may result in build errors
 - wxIntProperty::DoValidation() and wxFloatProperty::DoValidation() are
   no longer public since they are helpers intended for internal use only.
 
+- wxGridCellAttr ctor taking wxGridCellAttr pointer is now explicit.
+
 
 3.1.4: (released ????-??-??)
 ----------------------------

include/wx/generic/grid.h

@@ -428,16 +428,15 @@ public:
         Merged
     };
 
-    // ctors
+    // default ctor
-    wxGridCellAttr(wxGridCellAttr *attrDefault = NULL)
+    explicit wxGridCellAttr(wxGridCellAttr *attrDefault = NULL)
     {
         Init(attrDefault);
 
         SetAlignment(wxALIGN_INVALID, wxALIGN_INVALID);
     }
 
-    // VZ: considering the number of members wxGridCellAttr has now, this ctor
+    // ctor setting the most common attributes
-    //     seems to be pretty useless... may be we should just remove it?
     wxGridCellAttr(const wxColour& colText,
                    const wxColour& colBack,
                    const wxFont& font,

interface/wx/grid.h

@@ -912,7 +912,8 @@ public:
     /**
         Default constructor.
     */
-    wxGridCellAttr(wxGridCellAttr* attrDefault = NULL);
+    explicit wxGridCellAttr(wxGridCellAttr* attrDefault = NULL);
+
     /**
         Constructor specifying some of the often used attributes.
     */