Enable Open History Button by default.

The implementation plan is make it file extenstion sensitive,
enable only when *.sfg file selected.
Gtk::FileChooser::signal_selection_changed
etl::filename_extension(filename) == ".sfg"

synfig-studio/src/gui/app.cpp

@@ -2345,6 +2345,7 @@ App::dialog_open_file_with_history_button(const std::string &title, std::string
     dialog->set_current_folder(prev_path);
     dialog->add_button(Gtk::Stock::CANCEL, Gtk::RESPONSE_CANCEL);
     dialog->add_button(_("Open history"), RESPONSE_ACCEPT_WITH_HISTORY);
+    dialog->set_response_sensitive(RESPONSE_ACCEPT_WITH_HISTORY, true);
     dialog->add_button(Gtk::Stock::OPEN,   Gtk::RESPONSE_ACCEPT);
 
     // File filters