Improve tabline + buffer_min_count behaviour

Using ```BufEnter``` & ```BufUnload``` in place of ```CursorMoved``` there is no need to move the cursor to make the tabline appear when you have set ```buffer_min_count```.

For example, let's set ```buffer_min_count = 2```, then open a file and then open another file. With the previous behaviour you should have moved the cursor to see the tabline, now this is not requested anymore.