Add modified page to track which page is being modified. Useful in hybrid mode where underlying page can change at any time.
Point modified page to the loaded page when loading the current page. Modified page was always loaded when loading its page id. That blocked the history of the page to be loaded.
Sound dialogs also use the modified page.
Fix board history and side effects when changing page while dragging a sound.
Fix a crash when saving a board if nothing was done to it.