Don't show the top-level library categories as submenus

In the previous commit, the import library and example menus were made
recursive. The top level submenus here were the different kinds of
library (hardware-independent libraries, hardware-specific libraries and
sketchbook libraries). With this commit, these different kinds of
libraries are all put together at the top level, with (unselectable)
headers in between (like the boards menu, which already head these
headers for the different platforms).