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/edit.c

@@ -7741,6 +7741,8 @@ static void ins_mousescroll(int dir)
             (long)(curwin->w_botline - curwin->w_topline));
       else
         scroll_redraw(dir, 3L);
+    } else {
+        mouse_scroll_horiz(dir);
     }
     did_scroll = TRUE;
   }