Extend functionality of wxPropertyGridInterface::SetPropertyColoursToDefault method.

SetPropertyTextColour, SetPropertyBackgroundColour methods are able to set colours recursively for sub-properties but SetPropertyColoursToDefault method is not.
For the sake of consistency, SetPropertyColoursToDefault method is extended to have the same capabilities as SetPropertyTextColour and SetPropertyBackgroundColour. Behaviour and signature in default case (no recursion) is preserved.
For internal purposes there were also implemented helper methods in wxPGProperty class: SetDefaultColours, ClearCells.

include/wx/propgrid/property.h

@@ -2013,6 +2013,16 @@ public:
     void SetTextColour( const wxColour& colour,
                         int flags = wxPG_RECURSE );
 
+    /**
+        Sets property's default text and background colours.
+
+        @param flags
+            Default is wxPG_RECURSE which causes colours to be set recursively.
+            Omit this flag to only set colours for the property in question
+            and not any of its children.
+    */
+    void SetDefaultColours(int flags = wxPG_RECURSE);
+
     /** Set default value of a property. Synonymous to
 
         @code
@@ -2397,6 +2407,14 @@ protected:
                           FlagType ignoreWithFlags,
                           bool recursively );
 
+    /**
+        Clear cells associated with property.
+
+        @param recursively
+            If @true, apply this operation recursively in child properties.
+    */
+    void ClearCells(FlagType ignoreWithFlags, bool recursively);
+
     /**
         Makes sure m_cells has size of column+1 (or more).
     */