Code cleanup: Define wxPGProperty and wxPropertyGridPageState functions in their own class files

Having definitions of several wxPGProperty and wxPropertyGridPageState functions in wxPropertyGrid header is misleading so they should be moved to the files with their own classes definitions for the sake of clarity.

src/propgrid/propgridpagestate.cpp

@@ -2105,6 +2105,16 @@ void wxPropertyGridPageState::DoDelete( wxPGProperty* item, bool doDelete )
     VirtualHeightChanged();
 }
 
+bool wxPropertyGridPageState::IsDisplayed() const
+{
+    return (this == m_pPropGrid->GetState());
+}
+
+unsigned int wxPropertyGridPageState::GetActualVirtualHeight() const
+{
+    return DoGetRoot()->GetChildrenHeight(GetGrid()->GetRowHeight());
+}
+
 // -----------------------------------------------------------------------
 
 #endif  // wxUSE_PROPGRID