Fixed #4562 (single shared menu for all Shotwell views) and #4841 (sidebar and menu flash during import).

With this change the LibraryWindow's UI Manager is shared by all pages in the window.  When you switch to any page, it swaps the menus and toolbars it needs into the shared UI manager, and then swaps them back out when you leave the page.

Many of the changes here were because pages were assuming that their widgets existed even when pages were not in view.  With these changes, pages can access widgets only when they are active.