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/doxy/mainpage.md

@@ -209,13 +209,24 @@ The `<stack>` tag
 -----------------
 
 The `<stack>` tag groups elements that represent a single entry in an array.
-It can contain any elements except for nested `<stack>` elements.
+It can contain any elements except for nested `<stack>` or `<pane>` elements.
 
 The `<stack>` tag accepts the following attributes:
 
 * `pages` - The initial number of pages in the stack. This can also be
 set programmatically.
 
+The `<pane>` tag
+----------------
+
+The `<pane>` tag groups elements into a scrollable subpane.
+It can contain any elements except for nested `<stack>` or `<pane>` elements.
+
+The `<pane>` tag accepts the following attributes:
+
+* `framed` - Specifies whether a frame is drawn around the pane. Can
+be either `true` or `false`; defaults to `true`.
+
 Keyboard Shortcuts
 ------------------