another low-level input overhaul: split out low-level mouse input,

another low-level input overhaul: split out low-level mouse input,
remove references to ncurses key values from the global shortcut list
and restructure it to specify function key values when needed, hook up
verbatim input to the edit window, and do a few miscellaneous cleanups

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1596 35c25a1d-7b9e-4130-9fde-d3aeb78583b8