option editor: allow EntryOption to have a list of valid chars (not just invalid)

gtk2_ardour/option_editor.cc

@@ -347,9 +347,21 @@ void
 EntryOption::filter_text (const Glib::ustring&, int*)
 {
 	std::string text = _entry->get_text ();
+
+	if (!_valid.empty()) {
+		for (std::string::const_iterator t = text.begin(); t != text.end(); ) {
+			if (_valid.find_first_of (*t) == std::string::npos) {
+				t = text.erase (t);
+			} else {
+				++t;
+			}
+		}
+	}
+
 	for (size_t i = 0; i < _invalid.length(); ++i) {
 		text.erase (std::remove(text.begin(), text.end(), _invalid.at(i)), text.end());
 	}
+
 	if (text != _entry->get_text ()) {
 		_entry->set_text (text);
 	}

gtk2_ardour/option_editor.h

@@ -278,6 +278,7 @@ public:
 	void add_to_page (OptionEditorPage*);
 	void set_sensitive (bool);
 	void set_invalid_chars (std::string i) { _invalid = i; }
+	void set_valid_chars (std::string i) { _valid = i; }
 
 	Gtk::Widget& tip_widget() { return *_entry; }
 
@@ -291,6 +292,7 @@ private:
 	Gtk::Label* _label; ///< UI label
 	Gtk::Entry* _entry; ///< UI entry
 	std::string _invalid;
+	std::string _valid;
 };