Expose GUI ordering API

[brentyi]

1. We move away from setting order base on time.time(), which I imagine would fail in Windows. In Windows, time.time() uses 16ms increments; we've replaced this with a global counter.

2. We expose the .order property for GUI handles and the order= argument for "add GUI" methods. These will make it easier to, as an example, add a new GUI element between two existing ones or in place of an existing one.

[kerrj]

To add an element between one you would have to manually update the orders of all elements right? What happens if you accidentally specify the same order

[brentyi]

Like between two existing GUI elements?

If you have gui_a and gui_b, you could create a new GUI element at order=(gui_a.order + gui_b.order) / 2.0. The global counter's an integer but the order argument will take floats.

What happens if you accidentally specify the same order

I'm fairly confident that elements will just follow insertion order, but I think we can consider this undefined behavior.

[kerrj]

nice