Add a PagePanel container for Web projects

Add a PagePanel container to Web projects so applications can build multi-page interfaces inside one page — wizards, stepped forms, setup flows, and master-detail panes — where exactly one page is visible at a time and code switches pages as the user progresses.

Pages should be manageable in the visual designer, and switching the visible page in code or from the browser raises an event so applications can react. This would bring Web projects to parity with the desktop PagePanel and replace the current workarounds of hiding controls or misusing a TabPanel.