Refactor: display file selector dialog from within member function in wxFileProperty

By moving the code to display file selector dialog from wxPGFileDialogAdapter to wxFileProperty we can encapsulate the operation of showing the dialog because all required parameters are stored in the corresponding data members and there is no need to use call generic GetAttribute() function to retrieve them. This also helps in making wxFileProperty attributes built-ones in the future.

include/wx/propgrid/props.h

@@ -584,6 +584,8 @@ public:
     wxFileName GetFileName() const;
 
 protected:
+    bool DisplayEditorDialog(wxPropertyGrid* propGrid, wxString& value);
+
     wxString    m_wildcard;
     wxString    m_basePath; // If set, then show path relative to it
     wxString    m_initialPath; // If set, start the file dialog here