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. :^)
This is not only easier to comprehend code-wise and avoids some function
overloads, but also makes resizing the board while preserving game state
*a lot* easier. We now no longer have to allocate a new board on every
resize, we just grow/shrink the individual row vectors.
Also fixes a crash when clicking the board widget outside of the drawn
board area.
