Proposed changes to ListView

`ListView` is very difficult to understand. I believe this is because of the `Adapter`. Obfuscating the `Adapter` class & properties would make listview much simpler and easy to use. `AliasProperty` will be used so that LV is compatible with kvlang. No more using [this workaround](http://stackoverflow.com/questions/17100499/kivy-why-does-listview-declare-differently-in-kv-file).