This implements the rotate cw/ccw actions in PDFViewer.
Since the rendered pages are stored in a HashMap for caching,
the bitmap is wrapped in a struct with the current rotation.
This way the caching works as expected while zooming, and a new bitmap
is rendered when the page is rotated.
Applications previously had to create a GUI::Menubar object, add menus
to it, and then call GUI::Window::set_menubar().
This patch introduces GUI::Window::add_menu() which creates the menubar
automatically and adds items to it. Application code becomes slightly
simpler as a result. :^)
