Add About view.
Add visibility manager.
Add root view.
Add About window.
Remove render stats from the main window.
Add a module for identifiers used by the UI.
Fix some modification years.
Add application name to the About window.
Control main window visibility via VisibilityManager, quit when main window is closed.
Add window titles.
Fix About window title.
Remove accidental duplicate of MainWindow.
Verify that all updaters are unregistered before exiting normally.
Change updater exit error to RuntimeError.
Unregister all updaters before exit.
Support invisible UI and make sure that RootView cleans up all windows before exit.
Merge pull request #104 from kagu/multiple_windows2

Multiple windows