tui: Don't use 'pastetoggle' for automatic pasting

Add a new special key that can be used by UIs to toggle the 'paste' option and
use it in the TUI instead of the user's 'pastetoggle' value.

Close #2843 #2092

src/nvim/keymap.c

@@ -284,6 +284,7 @@ static struct key_name_entry {
   {K_SNR,             (char_u *)"SNR"},
   {K_PLUG,            (char_u *)"Plug"},
   {K_CURSORHOLD,      (char_u *)"CursorHold"},
+  {K_PASTE,           (char_u *)"Paste"},
   {0,                 NULL}
 };