Kivy Inspector selects wrong widgets

When trying to use Kivy Inspector, we noticed that it regularly selected widgets that were not visible on the screen. I found that the problem seems to be that Inspector loops through the widgets in order, which means it looks at the widgets in the very back first. I reversed the loops (in `highlight_at` and `pick`) so they look at the front-most children first, which seems to be working great.

I also made `pick` check for a `visible` property, and changed the code in `update_widget_graphics` to use the `gscale.xyz` property instead of `gscale.scale` to avoid warning spam.