Separated logic from presentation, lowercased file names

I pulled the code for generating the menu out into its own module called
overlay_page. I pulled the code for generating a menu, using
overlay_page, out into its own module called show_keyboard_shortcuts.
The logic for getting a list of keyboard shortcuts pertinent to the
current operating system is by itself in the module
get_editor_keyboard_shortcuts. This should make it easy for anyone to
change the appearance of the menu without worrying about breaking the
code which fetches the data.