Make wxPGProperty an abstract class

wxPGProperty is intended to be only a base class for property classes and therefore shouldn't be instantiated directly.

docs/changes.txt

@@ -120,6 +120,8 @@ Changes in behaviour which may result in build errors
   but must be explicitly selected (and libstdc++ must be installed in order
   to use it).
 
+- wxPGProperty ctors are not longer public since this class is intended to be
+  a base class and should not be instantiated directly.
 
 3.1.3: (released 2019-??-??)
 ----------------------------

include/wx/propgrid/property.h

@@ -979,14 +979,6 @@ class WXDLLIMPEXP_PROPGRID wxPGProperty : public wxObject
 public:
     typedef wxUint32 FlagType;
 
-    // Default constructor.
-    wxPGProperty();
-
-    // Constructor.
-    // All non-abstract property classes should have a constructor with
-    // the same first two arguments as this one.
-    wxPGProperty( const wxString& label, const wxString& name );
-
     // Virtual destructor.
     // It is customary for derived properties to implement this.
     virtual ~wxPGProperty();
@@ -1897,6 +1889,13 @@ public:
 
 protected:
 
+    // Ctors are ptotected because wxPGProperty is only a base class
+    // for all property classes and shouldn't be instantiated directly.
+    wxPGProperty();
+    // All non-abstract property classes should have a constructor with
+    // the same first two arguments as this one.
+    wxPGProperty(const wxString& label, const wxString& name);
+
     // Sets property cell in fashion that reduces number of exclusive
     // copies of cell data. Used when setting, for instance, same
     // background colour for a number of properties.

interface/wx/propgrid/property.h

@@ -401,8 +401,9 @@ wxPG_PROP_CLASS_SPECIFIC_3          = 0x00400000
 /**
     @class wxPGProperty
 
-    wxPGProperty is base class for all wxPropertyGrid properties. In
+    wxPGProperty is base class for all wxPropertyGrid properties and as such
-    sections below we cover few related topics.
+    it is not intended to be instantiated directly.
+    In sections below we cover few related topics.
 
     @li @ref pgproperty_properties
     @li @ref pgproperty_creating
@@ -848,37 +849,6 @@ class wxPGProperty : public wxObject
 public:
     typedef wxUint32 FlagType;
 
-    /**
-        Default constructor.
-    */
-    wxPGProperty();
-
-    /**
-        Constructor.
-        Non-abstract property classes should have constructor of this style:
-
-        @code
-
-        MyProperty( const wxString& label, const wxString& name, const T& value )
-            : wxPGProperty(label, name)
-        {
-            // Generally recommended way to set the initial value
-            // (as it should work in pretty much 100% of cases).
-            wxVariant variant;
-            variant << value;
-            SetValue(variant);
-
-            // If has private child properties then create them here.
-            // For example:
-            //     AddPrivateChild( new wxStringProperty("Subprop 1",
-            //                                           wxPG_LABEL,
-            //                                           value.GetSubProp1()));
-        }
-
-        @endcode
-    */
-    wxPGProperty( const wxString& label, const wxString& name );
-
     /**
         Virtual destructor. It is customary for derived properties to implement this.
     */
@@ -2080,6 +2050,40 @@ public:
     void*                       m_clientData;
 
 protected:
+
+    /**
+        Default constructor. It is protected because wxPGProperty is only
+        a base class for other property classes.
+    */
+    wxPGProperty();
+
+    /**
+        Constructor. It is protected because wxPGProperty is only a base
+        class for other property classes.
+        Non-abstract property classes should have constructor of this style:
+
+        @code
+
+        MyProperty( const wxString& label, const wxString& name, const T& value )
+            : wxPGProperty(label, name)
+        {
+            // Generally recommended way to set the initial value
+            // (as it should work in pretty much 100% of cases).
+            wxVariant variant;
+            variant << value;
+            SetValue(variant);
+
+            // If has private child properties then create them here.
+            // For example:
+            //     AddPrivateChild( new wxStringProperty("Subprop 1",
+            //                                           wxPG_LABEL,
+            //                                           value.GetSubProp1()));
+        }
+
+        @endcode
+    */
+    wxPGProperty( const wxString& label, const wxString& name );
+
     /**
         Sets property cell in fashion that reduces number of exclusive
         copies of cell data. Used when setting, for instance, same