This commit is a preparation step for some menu cleanup ahead. To make it
easier to understand which types of MenuItems are added where and when, I
converted the overloaded FbTk::Menu::insert() functions into explicit ones
(Menu::insertSubmenu(), Menu::insertItem(), etc. ). This makes it possible
to just grep for 'insertSubmenu()'.
Side effect this commit: it trims down the very verbose creation of menu items
in regards to how the labels are created.
Minor: FbTk::Menu::shown and FbTk::Menu::s_focused are moved out of the class.
