input: Remove CURSORHOLD key

Refactor input.c, normal.c and edit.c to use the K_EVENT special key to trigger
the CURSORHOLD event. In normal and edit mode, K_EVENT is treated as
K_CURSORHOLD, which enables better handling of arbitrary actions in those
states(eg: In normal mode the previous operator counts will be restored).

Also fix a test in vim_spec.lua. The test had a wrong assumption: cmdheight is
only used to determine when the press enter screen will be shown, not to limit
how many lines or control pagination.

src/nvim/keymap.c

@@ -283,7 +283,6 @@ static struct key_name_entry {
   {K_ZERO,            (char_u *)"Nul"},
   {K_SNR,             (char_u *)"SNR"},
   {K_PLUG,            (char_u *)"Plug"},
-  {K_CURSORHOLD,      (char_u *)"CursorHold"},
   {K_PASTE,           (char_u *)"Paste"},
   {0,                 NULL}
 };