New scroll pane control in the dialog engine

Also:
- Use a scroll pane for the credits in the about dialog, so that new credits can be added without altering other elements
- Use a scroll pane for the list of sections to be deleted in the resize outdoors dialog
- New cContainer superclass for controls that contain other controls
- Scrollbars consider their maximum as part of their state

src/classes/location.cpp

@@ -96,7 +96,7 @@ bool rectangle::contains(int x, int y) {
 rectangle_size_delegate::rectangle_size_delegate(rectangle& rect, int rectangle::* first, int rectangle::* last) :
 	forRect(rect), b1(first), b2(last) {}
 
-rectangle_size_delegate::operator int() {
+rectangle_size_delegate::operator int() const {
 	return forRect.*b2 - forRect.*b1;
 }
 
@@ -125,6 +125,26 @@ rectangle_size_delegate& rectangle_size_delegate::operator/=(int val) {
 	return *this;
 }
 
+rectangle_size_delegate& rectangle_size_delegate::operator=(const rectangle_size_delegate& val) {
+	return operator=(int(val));
+}
+
+rectangle_size_delegate& rectangle_size_delegate::operator+=(const rectangle_size_delegate& val) {
+	return operator+=(int(val));
+}
+
+rectangle_size_delegate& rectangle_size_delegate::operator-=(const rectangle_size_delegate& val) {
+	return operator-=(int(val));
+}
+
+rectangle_size_delegate& rectangle_size_delegate::operator*=(const rectangle_size_delegate& val) {
+	return operator*=(int(val));
+}
+
+rectangle_size_delegate& rectangle_size_delegate::operator/=(const rectangle_size_delegate& val) {
+	return operator/=(int(val));
+}
+
 rectangle_size_delegate rectangle::width() {
 	return rectangle_size_delegate(*this, &rectangle::left, &rectangle::right);
 }

src/classes/location.hpp

@@ -40,12 +40,17 @@ class rectangle_size_delegate {
 	int rectangle::* b2;
 	rectangle_size_delegate(rectangle& rect, int rectangle::* first, int rectangle::* last);
 public:
-	operator int();
+	operator int() const;
 	rectangle_size_delegate& operator=(int val);
 	rectangle_size_delegate& operator+=(int val);
 	rectangle_size_delegate& operator-=(int val);
 	rectangle_size_delegate& operator*=(int val);
 	rectangle_size_delegate& operator/=(int val);
+	rectangle_size_delegate& operator=(const rectangle_size_delegate& val);
+	rectangle_size_delegate& operator+=(const rectangle_size_delegate& val);
+	rectangle_size_delegate& operator-=(const rectangle_size_delegate& val);
+	rectangle_size_delegate& operator*=(const rectangle_size_delegate& val);
+	rectangle_size_delegate& operator/=(const rectangle_size_delegate& val);
 };
 
 struct rectangle {