Restrict resizing to visible screen

This makes sure that the debugbar isn't dragged to high, so the handler is unreachable, and not too low to make sure it's always clear that it's just resized, not broken/collapsed.
See https://github.com/barryvdh/laravel-debugbar/issues/122, https://github.com/barryvdh/laravel-debugbar/issues/171 and https://github.com/barryvdh/laravel-debugbar/issues/16