I was a bit confused by the fact that a method named `build_system_menu()`
first enumerates a directory. Moving the app/category discovery to a dedicated
function that returns the GUI-relevant information makes this process a bit
less surprising.
As an added bonus, `g_app_category_menus` was actually only a temporary mapping,
and didn't need to be global. In theory, SystemMenu should use a handful fewer
of bytes now.
And move canonicalized_path() to a static method on LexicalPath.
This is to make it clear that FileSystemPath/canonicalized_path() only
perform *lexical* canonicalization.
Previously opening the shutdown dialog and cancelling out of it would
cause SystemMenu to exit due to the exit-when-there-are-no-more-windows
mechanism in GUI::Application. Fix this by opting out of it.
