tui: Use underline cursor in Replace mode

This is a port of my original contribution to Vim, added in 7.4.687
(https://github.com/vim/vim/commit/v7-4-687). The TUI code has been
heavily refactored (see esp. 25ceadab37),
so this required some translation, but the logic is the same.

test/functional/ui/screen.lua

@@ -341,7 +341,7 @@ function Screen:_handle_mouse_off()
 end
 
 function Screen:_handle_mode_change(mode)
-  assert(mode == 'insert' or mode == 'normal')
+  assert(mode == 'insert' or mode == 'replace' or mode == 'normal')
   self._mode = mode
 end