mouse: Implement horizontal scroll. #3450

- Code from Vim source.
- Removed the check for 'guioptions'
- mouse_spec.lua: test <ScrollWheelLeft> and <ScrollWheelRight>
- Move horizontal scroll logic to mouse.c
- Remove 'gui_' from the function names
- Renamed variables to be more specific (as opposed to generic p, w).
- Marked some functions as `static`

src/nvim/normal.c

@@ -3927,6 +3927,8 @@ static void nv_mousescroll(cmdarg_T *cap)
       cap->count0 = 3;
       nv_scroll_line(cap);
     }
+  } else {
+    mouse_scroll_horiz(cap->arg);
   }
 
   curwin->w_redr_status = true;