Multi display update

[qcapen]

Restore multiple display instance updates. This solution leads to unnecessary redrawing whenever there is more than one display widget with the same display source, but in other situations adds no overhead.

[qcapen]

I messed around with adding a new Callback that only is called on update. The biggest issue is I could not get it to build outside of Kivy as the dependencies were not designed to be derived from in an outside project. This meant customizing Kivy which I really didn't want to get into in this release. May revisit in the near future.

[jabdoa2]

Cool. Maybe we can try to upstream the new callback on ask_update or how it's called? I will test this later on my machine.

[jabdoa2]

i need to test this again. last time it did not work for me